Enable installation_store authorize to fallback to bots (prep for #254)

[seratch]

This pull request improves InstallationStoreAuthorize and AsyncInstallationStoreAuthorize for better installation data resolution as part of #254 feature addition.

Even when all user installations are revoked, bot scope installation should exist as long as the app is not uninstalled in the workspace. This is the reason why this SDK recommends having two data tables - bots and user installations. This pull request improves the authorize functions to be capable to resolve bot installation regardless of the behavior of find_installation method in InstallationStore / AsyncInstallationStore. Refer to the test code for learning what the use case is.

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 #287 (052fbcb) into main (efab010) will decrease coverage by 0.02%.
The diff coverage is 80.95%.

❗ Current head 052fbcb differs from pull request most recent head deaba2e. Consider uploading reports for the commit deaba2e to get more accurate results

@@            Coverage Diff             @@
##             main     #287      +/-   ##
==========================================
- Coverage   91.50%   91.47%   -0.03%     
==========================================
  Files         160      160              
  Lines        5000     5010      +10     
==========================================
+ Hits         4575     4583       +8     
- Misses        425      427       +2     

Impacted Files	Coverage Δ
slack_bolt/authorization/authorize.py	86.45% <80.00%> (-0.50%)	⬇️
slack_bolt/authorization/async_authorize.py	86.73% <81.81%> (-0.23%)	⬇️

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