Admin APIs

[gmgigi96]

This PR adds an admin API to reva: an authenticated, audited way to look at and operate a running fleet from a single reva admin command. It is off by default and only enabled where a process is given an admin group.

The building block is a small control channel that every reva process runs beside its services. The admin reaches into a live instance through it to read state or run an operation (an invocation), resolving targets through the registry, so a command can hit one instance, all instances of a service, or the whole fleet, with the results merged.

On top of it, an operator can:

• inspect the fleet: list services and their health, read and diff configuration, read or follow logs, trace a single request or user across services, dump goroutines, and watch an instance's in-flight request activity;
• act on it: set a process's log level at runtime, take instances out of and back into rotation for maintenance, drive the background jobs runner (list and enqueue jobs, trigger or cancel runs), and impersonate a user.

On the same host it needs no login, it is reached over a Unix socket and authorized by the caller's OS identity; remotely, an operator first steps up to a short-lived admin token.

Jobs is the first example of the intended pattern: typed admin API methods whose implementation reuses the invoke channel underneath, fanning out to every instance for live state, or targeting one when the state is shared.