Add authorize function support #98

The authorize function that is available in Bolt JS is not yet supported in Bolt for Python.

# these arguments should be injected in correspondence with their names
# also, the Bolt framework provides `args` as a single value as with middleware/listeners
def authorize(enterprise_id, team_id, user_id):
    return {
        "bot_id": "B111",
        "bot_user_id": "W111",
        "bot_token": "xoxb-"
    }

app = App(
    signing_secret: os.environ["SLACK_SIGNING_SECRET"],
    authorize: authorize
)

For most use cases, probably, developers prefer using installation_store. That being said if a developer would like to implement a simple but more flexible way to fetch installation data, going with the authorize function can be a handy way.

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
• Others

Requirements

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