Add plugins endpoint to the REST API

[ephraimbuddy]

This PR seeks to add /plugins endpoint. The endpoint exposes the plugins loaded to the webserver

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[github-actions]

The PR most likely needs to run full matrix of tests because it modifies parts of the core of Airflow. However, committers might decide to merge it quickly and take the risk. If they don't merge it quickly - please rebase it to the latest master at your convenience, or amend the last commit of the PR, and push it with --force-with-lease.

[mik-laj]

Should we update summary of changes in docs?
http://apache-airflow-docs.s3-website.eu-central-1.amazonaws.com/docs/apache-airflow/latest/stable-rest-api-ref.html#section/Overview/Versioning-and-Endpoint-Lifecycle

[ephraimbuddy]

Should we update summary of changes in docs?
http://apache-airflow-docs.s3-website.eu-central-1.amazonaws.com/docs/apache-airflow/latest/stable-rest-api-ref.html#section/Overview/Versioning-and-Endpoint-Lifecycle

Sure. Will do that

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[ephraimbuddy]

Hi @kaxil, @turbaszek, @mik-laj please re-review this as I have added pagination to the endpoint

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The Workflow run is cancelling this PR. It has some failed jobs matching ^Pylint$,^Static checks,^Build docs$,^Spell check docs$,^Backport packages$,^Provider packages,^Checks: Helm tests$,^Test OpenAPI*.