Add tctl support for static host users

[atburke]

This change adds support in tctl for the static host user resource.

It also adds an internal StaticHostUser type to use where possible instead of the proto type. This makes working with the resource less painful for the end user. The new format is identical to role labels.

Original:

# ...
node_labels:
  Values:
    foo:
      Values:
        - bar

New:

# ...
node_labels:
  foo: bar

The new serialization is a breaking change in the backend. However, no part of static host users has been backported, so no one will be affected.

Part of #42712.

Changelog: Added StaticHostUser resource

[espadolini]

If this is only needed for user interaction, do we need to change all of our APIs and the backend serialization format?

How can we make sure we won't forget to update the user representation after changing the API?

Should IaC tooling be using the proto-side representation or the backend-side representation?

[atburke]

Replacing this with another PR after #46038.