DUX-3346: Support custom deserializers in request verification

[lf-]

This turns out to be necessary for the impressively messed up nonsense that Slack does with their interactive webhooks, which range from "json in a form parameter" (WHAT) to "entirely putting the thing in form parameters".

More code to support this may come in the future, but we need this foundation to be able to disentangle the bad code in mwb using this.

Sadly does a breaking change due to import Web.Slack.Experimental.RequestVerification (SlackVerificationFailed(..)) no longer importing constructors. The rest of this is not breaking changes.

Before submitting your PR, check that you've:

• [x] Documented new APIs with Haddock markup
• [x] Added @since declarations to the Haddock

After submitting your PR:

• [x] Update the Changelog.md file with a link to your PR
• [x] Bumped the version number if there isn't an (unreleased) on the Changelog
• [ ] Check that CI passes (or if it fails, for reasons unrelated to your change, like CI timeouts)

[linear[bot]]

DUX-3346 Add support to slack-web for dealing with verifying urlencoded request bodies