Automatic refresh of TLS client cert for pulsaradmin

[massakam]

Is your feature request related to a problem? Please describe.
PulsarAdmin in Java has a feature to periodically refresh loaded TLS client certificates (cf. apache/pulsar#8831), but Go does not seem to have a similar feature. Therefore, in environments where client certificates have a short validity period and are updated frequently, pulsaradmin that runs for a long time may be unable to access the Admin API.

Describe the solution you'd like
It would be nice if pulsaradmin could automatically refresh the client certificates it loaded. It would be even better if we could set the refresh interval like in Java.