kuma-cp: add generic versioner for xDS resources

[yskopets]

Summary

• add generic versioner for xDS resources

[jakubdyszkiewicz]

What do you think about more immutable version of this? In my opinion - the less mutability the easier is to reason about code. How about Version() instead of modifing snapshot, would return map[type]version which is then applied on snapshot outside of this method?

[yskopets]

@jakubdyszkiewicz I've changed Versioner interface to

// SnapshotVersioner assigns versions to xDS resources in a new Snapshot.
type SnapshotVersioner interface {
	Version(new, old Snapshot) Snapshot
}

[jakubdyszkiewicz]

thanks