Automatic refresh of TLS client cert for pulsaradmin

**Is your feature request related to a problem? Please describe.**
PulsarAdmin in Java has a feature to periodically refresh loaded TLS client certificates (cf. https://github.com/apache/pulsar/pull/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.

Start by comparing the Go pulsaradmin TLS client-certificate handling with the Java behavior described in Apache Pulsar pull request 8831. Define how a configurable refresh interval should work and consider the issue complete when a long-running pulsaradmin continues accessing the Admin API after its client certificates are replaced.

Written by the indexing model from the issue text.