Add custom function support

[seratch]

This pull request adds remote function support, which is still in beta, to this SDK. Sorry, the duplicated PRs (due to my operation error) are confusing but this is the final version of https://github.com/slackapi/java-slack-sdk/pull/1239

Developers can implement a remote function and its interactivity this way:

// app.event(FunctionExecutedEvent.class, (req, ctx) -> {
// app.function("hello", (req, ctx) -> {
app.function(Pattern.compile("^he.+$"), (req, ctx) -> {
  ctx.logger.info("req: {}", req);
  ctx.client().chatPostMessage(r -> r
    .channel(req.getEvent().getInputs().get("user_id").asString())
    .text("hey!")
    .blocks(asBlocks(actions(a -> a.blockId("b").elements(asElements(
      button(b -> b.actionId("remote-function-button-success").value("clicked").text(plainText("block_actions success"))),
      button(b -> b.actionId("remote-function-button-error").value("clicked").text(plainText("block_actions error"))),
      button(b -> b.actionId("remote-function-modal").value("clicked").text(plainText("modal view")))
    )))))
  );
  return ctx.ack();
});

app.blockAction("remote-function-button-success", (req, ctx) -> {
  Map<String, Object> outputs = new HashMap<>();
  outputs.put("user_id", req.getPayload().getFunctionData().getInputs().get("user_id").asString());
  ctx.client().functionsCompleteSuccess(r -> r
    .functionExecutionId(req.getPayload().getFunctionData().getExecutionId())
    .outputs(outputs)
  );
  ctx.client().chatUpdate(r -> r
    .channel(req.getPayload().getContainer().getChannelId())
    .ts(req.getPayload().getContainer().getMessageTs())
    .text("Thank you!")
  );
  return ctx.ack();
});
app.blockAction("remote-function-button-error", (req, ctx) -> {
  ctx.client().functionsCompleteError(r -> r
    .functionExecutionId(req.getPayload().getFunctionData().getExecutionId())
    .error("test error!")
  );
  ctx.client().chatUpdate(r -> r
    .channel(req.getPayload().getContainer().getChannelId())
    .ts(req.getPayload().getContainer().getMessageTs())
    .text("Thank you!")
  );
  return ctx.ack();
});
app.blockAction("remote-function-modal", (req, ctx) -> {
  ctx.client().viewsOpen(r -> r
    .triggerId(req.getPayload().getInteractivity().getInteractivityPointer())
    .view(view(v -> v
      .type("modal")
      .callbackId("remote-function-view")
      .title(viewTitle(vt -> vt.type("plain_text").text("Remote Function test")))
      .close(viewClose(vc -> vc.type("plain_text").text("Close")))
      .submit(viewSubmit(vs -> vs.type("plain_text").text("Submit")))
      .notifyOnClose(true)
      .blocks(asBlocks(input(input -> input
        .blockId("text-block")
        .element(plainTextInput(pti -> pti.actionId("text-action").multiline(true)))
        .label(plainText(pt -> pt.text("Text").emoji(true)))
      )))
    )));
  ctx.client().chatUpdate(r -> r
    .channel(req.getPayload().getContainer().getChannelId())
    .ts(req.getPayload().getContainer().getMessageTs())
    .text("Thank you!")
  );
  return ctx.ack();
});

app.viewSubmission("remote-function-view", (req, ctx) -> {
  Map<String, Object> outputs = new HashMap<>();
  outputs.put("user_id", ctx.getRequestUserId());
  ctx.client().functionsCompleteSuccess(r -> r
    .functionExecutionId(req.getPayload().getFunctionData().getExecutionId())
    .outputs(outputs)
  );
  return ctx.ack();
});
app.viewClosed("remote-function-view", (req, ctx) -> {
  Map<String, Object> outputs = new HashMap<>();
  outputs.put("user_id", ctx.getRequestUserId());
  ctx.client().functionsCompleteSuccess(r -> r
    .functionExecutionId(req.getPayload().getFunctionData().getExecutionId())
    .outputs(outputs)
  );
  return ctx.ack();
});

References:

• https://github.com/slackapi/bolt-python/pull/986

Category (place an x in each of the [ ])

• [x] bolt (Bolt for Java)
• [ ] bolt-{sub modules} (Bolt for Java - optional modules)
• [x] slack-api-client (Slack API Clients)
• [ ] slack-api-model (Slack API Data Models)
• [ ] slack-api-*-kotlin-extension (Kotlin Extensions for Slack API Clients)
• [x] slack-app-backend (The primitive layer of Bolt for Java)

Requirements

Please read the Contributing guidelines and Code of Conduct before creating this issue or pull request. By submitting, you agree to those rules.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 71.73913% with 13 lines in your changes missing coverage. Please review.

Project coverage is 74.96%. Comparing base (efa3f24) to head (9dd4118). Report is 1 commits behind head on main.

Files	Patch %	Lines
bolt/src/main/java/com/slack/api/bolt/App.java	57.14%	2 Missing and 4 partials :warning:
...m/slack/api/bolt/request/builtin/EventRequest.java	57.14%	0 Missing and 3 partials :warning:
...ck/api/bolt/request/builtin/ViewClosedRequest.java	33.33%	1 Missing and 1 partial :warning:
.../main/java/com/slack/api/bolt/context/Context.java	83.33%	0 Missing and 1 partial :warning:
...k/api/bolt/request/builtin/BlockActionRequest.java	85.71%	0 Missing and 1 partial :warning:

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1241      +/-   ##
============================================
- Coverage     74.99%   74.96%   -0.03%     
- Complexity     4190     4202      +12     
============================================
  Files           452      453       +1     
  Lines         12931    12975      +44     
  Branches       1331     1342      +11     
============================================
+ Hits           9697     9727      +30     
- Misses         2463     2467       +4     
- Partials        771      781      +10     

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