Fix #197 by supporting classic bot messages in app.message constraints

[seratch]

This pull request fixes #197 by adding support for classic app's bot messages in app.message listener constraints.

When an app with the current permission model posts a message in a channel, Events API sends the following payload (Payload A) to your app. On the other hand, if a message is posted by a classic app, the Event API payload (Payload B) can be a bit different. The notable difference is the existence of "bot_message" subtype. Bolt for Python had not been including this pattern for the events that can be handled by app.message listeners. This pull request resolves this issue.

Payload A

current_bot_message_event_payload = {
    "team_id": "T111",
    "enterprise_id": "E111",
    "api_app_id": "A111",
    "event": {
        "type": "message",
        "team": "T111",
        "channel": "C111",
        "channel_type": "channel",
        "ts": "1610261539.000900",
        "text": "Hi there! Thanks for sharing the info!",
        "user": "UB111",
        "bot_id": "B999",
        "bot_profile": {},
        "event_ts": "1610261539.000900",
    },
    "type": "event_callback",
    "event_id": "Ev222",
    "event_time": 1610261539,
    "authorizations": [],
    "is_ext_shared_channel": False,
    "event_context": "1-message-T111-C111",
}

Payload B

classic_bot_message_event_payload = {
    "team_id": "T111",
    "enterprise_id": "E111",
    "api_app_id": "A111",
    "event": {
        "type": "message",
        "subtype": "bot_message",
        "channel": "C111",
        "channel_type": "channel",
        "ts": "1610262363.001600",
        "text": "Hi there! Thanks for sharing the info!",
        "username": "classic-bot",
        "bot_id": "B888",
        "event_ts": "1610262363.001600",
    },
    "type": "event_callback",
    "event_id": "Ev333",
    "event_time": 1610262363,
    "authorizations": [],
    "is_ext_shared_channel": False,
    "event_context": "1-message-T111-C111",
}

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

• slack_bolt.App and/or its core components
• slack_bolt.async_app.AsyncApp and/or its core components
• Adapters in slack_bolt.adapter
• Document pages under /docs
• Others

Requirements (place an x in each [ ])

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

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've run ./scripts/install_all_and_run_tests.sh after making the changes.

[codecov]

Codecov Report

Merging #198 (6da54bc) into main (512a53c) will decrease coverage by 0.02%.
The diff coverage is 93.33%.

@@            Coverage Diff             @@
##             main     #198      +/-   ##
==========================================
- Coverage   92.14%   92.12%   -0.03%     
==========================================
  Files         149      149              
  Lines        4496     4508      +12     
==========================================
+ Hits         4143     4153      +10     
- Misses        353      355       +2     

Impacted Files	Coverage Δ
slack_bolt/listener_matcher/builtins.py	93.53% <91.66%> (-0.71%)	⬇️
slack_bolt/app/app.py	86.78% <100.00%> (+0.03%)	⬆️
slack_bolt/app/async_app.py	94.39% <100.00%> (+0.01%)	⬆️

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[seratch]

It seems no one has concerns on this change. Let me merge this now.