Inspect a channel's alias, if any (operator)

Declares one release channel to be an alias of another. When set, the api resolves every request for the alias channel (manifest resolution + the operator rollout ops) to the target channel's state, so a client that still sends the old name keeps working after a rename. The alias name is the {channel} path segment; the body carries only target. See design doc release-channel-aliasing-and-selection.md in link/mablnet/docs.