Include VC logs that indicate exited validators

[philknows]

Problem description

When users perform a voluntary validator exit, there is no visible indication of the exit in the validator client. Currently it just ceases to publish attestations, but no indication that its status has changed.

Solution description

We should log changes in validator status. If it's exiting and if it's exited. If it's exited, we could probably remind users here that validator X is exited and give a status update. It may even be worth exploring the validator lifecycle and periodically log the status of validators within the VC. IIRC we only export the status (pending, active) on startup.

Additional context

See suggestion from Sea Monkey in discord: https://discord.com/channels/593655374469660673/593655641445367808/1463162836967624714