Enable to use installation_store_bot_only flag not only as the top-level arg #177

With version 1.1.2, the newly added flag installation_store_bot_only works only when being set to the top-level field. The constructor's initialization needs to be smarter to check if the flag is set in the OAuthFlow / OAuthSettings level.

Reproducible in:

pip freeze | grep slack
python --version
sw_vers && uname -v # or `ver`

The slack_bolt version

version 1.1.2

Python runtime version

any

OS info

any

Steps to reproduce:

App(installation_store_bot_only=True)
App(oauth_flow=OAuthFlow(settings=OAuthSettings(installation_store_bot_only=True))
App(oauth_settings=OAuthSettings(installation_store_bot_only=True))

Expected result:

All the three examples should work in the same way.

Actual result:

Only the first one works as expected.

Requirements

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