Fix #183 Easier way to configure the direct install URL on App Directory

[seratch]

See #183 for the context and details.

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.

[seratch]

I will rerun the build after merging #187

[codecov]

Codecov Report

Merging #186 (29b5254) into main (899ce76) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #186      +/-   ##
==========================================
+ Coverage   92.13%   92.14%   +0.01%     
==========================================
  Files         149      149              
  Lines        4488     4496       +8     
==========================================
+ Hits         4135     4143       +8     
  Misses        353      353              

Impacted Files	Coverage Δ
slack_bolt/oauth/async_oauth_flow.py	91.30% <100.00%> (+0.15%)	⬆️
slack_bolt/oauth/async_oauth_settings.py	100.00% <100.00%> (ø)
slack_bolt/oauth/oauth_flow.py	90.43% <100.00%> (+0.16%)	⬆️
slack_bolt/oauth/oauth_settings.py	100.00% <100.00%> (ø)

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[seratch]

To reviewers, please let me know if you see something unclear for reviewing this (checking #183 should be helpful for learning the context and the reason behind this change). Considering the schedule for next minor version release, I would like to merge this change by the end of this week in my timezone.