Add v2 static host user resource

[atburke]

This change adds the v2 static host user resource.

In the v1 resource, more than one SHU resource with the same login but with conflicting other fields (groups, uid/gid, etc) could apply to a node. The host user's expected configuration would be ambiguous and sensitive to the order in which the resources were created.

In the v2 resource, only one resource can exist per login (the login is now the resource name to enforce this). The SHU resource contains a list of matchers to apply different configurations to different nodes:

kind: static_host_user
metadata:
  name: alice
spec:
  matchers:
    - node_labels:
        - name: foo
          values: ["bar"]
      groups: ["foo", "bar"]
      # sudoers, uid/gid, etc.

If more than one matcher applies to the node, the node can reject the host user without needing to know about any other SHU resources.

Static host users aren't in any releases yet, so no need to worry about breaking changes.

Part of #42712.

[zmb3]

Did we ever release the V1 resource? If not, can we make these changes without introducing a V2?

[rosstimothy]

Did we ever release the V1 resource? If not, can we make these changes without introducing a V2?

The proto linter makes no exceptions for unreleased protos. The only way to achieve that would be to delete v1 and add it back.

[rosstimothy]

Will we be deleting v1 since it's not intended to be used and was never released?