Reorganize some commands under "debusine-admin worker"

This is basically yak-shaving, but hopefully in a useful direction. I was thinking about using worker activation tokens to make the integration tests faster and more reliable (because they wouldn't need to use a sleep loop to wait for a worker token to appear in order to enable it), but that will probably need a new debusine-admin command to register a pre-activated worker; and I didn't really want to add a new command when the rough consensus seems to be that we want to reorganize the existing commands into a noun-then-verb structure.

We can remove the old command names at some point, probably after the next major Debusine release.