Extract post authenticate tasks from CallbackController

[zzooeeyy]

What this PR does

• https://github.com/Shopify/develop-app-access/issues/235

• Extract common logic to run post authenticate task from CallbackController so that it can be used after the token exchange flow.

• Added PostAuthenticateTasks to run the authenticated task

• This is configurable as config.custom_post_authenticate_tasks, if that's not set, the default config.post_authenticate_tasks will return PostAuthenticateTasks class

• Updated docs on post authenticate tasks

• Added deprecation warnings

Tophatting:

Deprecation warning in server start from configuration

Deprecation reminder in tests logs

When config.custom_post_authenticate_tasks is set, it will be used to handle post authenticate tasks

• I also tested not setting the custom_post_authenticate_tasks to ensure the methods in CallbackController to perform post authenticate tasks are done.

Checklist

Before submitting the PR, please consider if any of the following are needed:

• [x] Update CHANGELOG.md if the changes would impact users
• [x] Update README.md, if appropriate.
• [x] Update any relevant pages in /docs, if necessary
• [x] For security fixes, the Disclosure Policy must be followed.