Enable developers to customize the "/slack/install" webpage content

[seratch]

Description

As mentioned at #975, there is no way to directly redirect installers to the authorize url with the out-of-the-box receivers. Also, the code that generates the /slack/install endpoint is not yet customizable for developers. We can enhance all the built-in receivers to accept a function that render "/slack/install" web page content as part of installerOptions.

As reference, other Bolt frameworks enables this in the following ways:

• Bolt for Java accepts a renderer object in AppConfig: https://github.com/slackapi/java-slack-sdk/blob/v1.8.1/bolt/src/main/java/com/slack/api/bolt/AppConfig.java#L148-L153
• Bolt for Python allows to override the renderer method in the OAuth flow class: https://github.com/slackapi/bolt-python/blob/v1.6.1/slack_bolt/oauth/oauth_flow.py#L194-L195

Note that this task is related to #977.

What type of issue is this? (place an x in one of the [ ])

• bug
• enhancement (feature request)
• question
• documentation related
• example code related
• testing related
• discussion

Requirements (place an x in each of the [ ])

• 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 searched for any related issues and avoided creating a duplicate issue.