Manager should expose http.Handlers to allow extension by users

Based on a conversation we had on the community call on the 18th June 2020:

There are several http.Handlers set up by the manager that are currently only accessible if served by the manager (metrics and webhooks). If the manager exposed these handlers once constructed this would allow extension by authors to customise how these endpoints are served.

For example, an author could use their own server and serve TLS connections directly instead of using the HTTP only server that is currently implemented with controller-runtime

Or the user could add some HTTP middleware to ensure that all requests to the endpoints are logged in a particular way or authenticated and authorized before serving the content