Add --disable flag to "headscale nodes expire" CLI command and
disable_expiry field handling in the gRPC API to allow disabling
key expiry for nodes. When disabled, the node's expiry is set to
NULL and IsExpired() returns false.
The CLI follows the new grpcRunE/RunE/printOutput patterns
introduced in the recent CLI refactor.
Also fix NodeSetExpiry to persist directly to the database instead
of going through persistNodeToDB which omits the expiry field.
Fixes#2681
Co-authored-by: Marco Santos <me@marcopsantos.com>
248d240186