Add Socket Mode support #159

[seratch]

This pull request is still work in progress. The details can be changed until the GA release (v1.2.0)

#159

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-io]

Codecov Report

Merging #160 (3833bf3) into main (1d01c67) will decrease coverage by 4.20%.
The diff coverage is 0.45%.

@@            Coverage Diff             @@
##             main     #160      +/-   ##
==========================================
- Coverage   91.72%   87.51%   -4.21%     
==========================================
  Files         149      159      +10     
  Lines        4434     4630     +196     
==========================================
- Hits         4067     4052      -15     
- Misses        367      578     +211     

Impacted Files	Coverage Δ
slack_bolt/adapter/socket_mode/__init__.py	0.00% <0.00%> (ø)
slack_bolt/adapter/socket_mode/aiohttp/__init__.py	0.00% <0.00%> (ø)
...ack_bolt/adapter/socket_mode/async_base_handler.py	0.00% <0.00%> (ø)
slack_bolt/adapter/socket_mode/async_handler.py	0.00% <0.00%> (ø)
slack_bolt/adapter/socket_mode/async_internals.py	0.00% <0.00%> (ø)
slack_bolt/adapter/socket_mode/base_handler.py	0.00% <0.00%> (ø)
slack_bolt/adapter/socket_mode/builtin/__init__.py	0.00% <0.00%> (ø)
slack_bolt/adapter/socket_mode/internals.py	0.00% <0.00%> (ø)
...t/adapter/socket_mode/websocket_client/__init__.py	0.00% <0.00%> (ø)
...ck_bolt/adapter/socket_mode/websockets/__init__.py	0.00% <0.00%> (ø)
... and 22 more

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