⚠ Introduce Metrics Options struct & secure metrics serving

[sbueringer]

This PR introduces metrics options on the manager options. Via the new metrics options it will now be possible to serve metrics via tls and to use authentication and authorization.

• Metrics.FilterProvider allows to provide a metrics filter.
  • filters.WithAuthenticationAndAuthorization is a builtin filter that implements authentication and authorization
    • Please note: consumers of controller-runtime will only get the dependency to k8s.io/apiserver if they actually import/use this filter. Here is an example repo to show that the go.mod dependencies don't change with this PR except if the filter is configured (https://github.com/sbueringer/controller-runtime-tests/tree/master/metrics-auth)
• Metrics.SecureServing enables serving metrics on https
  • This is necessary for the above filter because tokens for authentication should not be sent via http
• Metrics.ExtraHandlers makes it possible to specify the extra handlers directly on the options. manager.AddMetricsExtraHandler has been removed.

Fixes #2073

[sbueringer]

@alvaroaleman @vincepri
I think this should be ready for a first round of review & discussion.

After that I'll add more documentation and add / fix unit tests.

[sbueringer]

@alvaroaleman Thx for the review. Changed the defaulting behavior and added test coverage

[sbueringer]

@vincepri PTAL when you have some time :)

[sbueringer]

Refactored the implementation to avoid introducing a dependency to k8s.io/apiserver for folks who are not using the authentication/authorization