Metadata-only list/watch support

[DirectXMan12]

The kube apiserver supports listing and watching just the metadata for resources. We should expose support for this both in our watch machinery and in our client.

This is particularly useful for performance when watching large numbers of different resources, or when you're watching a particularly numerous resource but need only the metadata for most/all instances of the resource.

/kind feature
/priority important-longterm

[DirectXMan12]

(FWIW, I'm working on this right now)

[vincepri]

Thanks @DirectXMan12 ! Let me know if I can be of any help, I worked on a specialized client in Cluster API for this, it'd be awesome to get support in the controller runtime client :)

[DirectXMan12]

Oooh, I'll send it over to you for review then. I've got most of a PR, just need to finish tests (got distracted by some other stuff since I filed this ;-) )