Improve the logic to extract the requestUserId for context objects

[seratch]

While testing this new feauture, I found that the logic to set context.requestUserId can be improved for Events API patterns. I will make changes for it in a different pull request.

As I mentioned at https://github.com/slackapi/java-slack-sdk/pull/1144, bolt-java's user ID extration for context objects can be improved like bolt-python does.

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

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

Requirements

Please make sure if this topic is specific to this SDK. For general questions/issues about Slack API platform or its server-side, could you submit questions at https://my.slack.com/help/requests/new instead. :bow:

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