create undocumented message bot events

[bruceadowns]

• create undocumented message bot events
• message_bot_add
• message_bot_remove
• these occur on add/remove of incoming webhook

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

• 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

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[bruceadowns]

@seratch I would like to add these 2 events to my bot, would you pls review. Also, I suspect you may want unit tests, if so pls give me an overview of what tests you’d like to see, and where they go.

[seratch]

Thanks for taking the time to send this pull request. Let me mention two points:

• It seems these two subtypes may actually exist but it's not officially listed in the document yet; so at this moment, we're unable to add these changes to SDKs. Our team is communicating with relevant teams on this, so once it's available in the public documents, we can consider adding these classes to this SDK
• This pull request includes extra changes that are not related to the message event class addition; some of the extra changes seem to be beneficial for this SDK's quality. If you're fine to spend more time, could you revert those changes in this PR and make a different PR for them? We can review it separately