Decouple webhook and manager packages

[adrienjt]

Context

I'm currently integrating multicluster-controller with controller-runtime's webhook package, which, unfortunately, depends on the manager package.

Multicluster-controller doesn't use controller-runtime's manager package, but a more lightweight version that only orchestrates runnables (caches and controllers), while the cluster dependencies are extracted into multiple Cluster structs.

Problem

Controller-runtime's webhook package depends on the manager.Manager interface, even though it only needs a fraction of it, namely Add(Runnable) for the server, and GetScheme() and GetRESTMapper() for the builder.

I could create an ad-hoc implementation of manager.Manager on my side, with lots of panics in the methods webhook doesn't need, forwarding the three that are actually needed to a multicluster-controller manager and cluster, but I believe there is a cleaner solution, which could also benefit others.

Proposed Solution

Define small interfaces where they're needed. They will be implemented implicitly by controller-runtime's controllerManager struct, and could be implemented by third-party packages.

// in pkg/webhook/server.go
type Manager interface {
	Add(Runnable) error
} 

// in pkg/webhook/admission/builder/builder.go
type Manager interface {
	GetScheme() *runtime.Scheme
	GetRESTMapper() meta.RESTMapper
} 

It should also make testing easier, and would be arguably more idiomatic: https://blog.chewxy.com/2018/03/18/golang-interfaces/

If you agree to this proposed change, I can submit a PR.

[DirectXMan12]

/kind feature
/priority important-longterm

Yeah, I'd be happy to accept a PR fixing this -- it makes sense to me to declare exactly what's needed.

[DirectXMan12]

mostly solved by #323