[improve] Make ZTS proxy configurable in athenz auth plugin

[massakam]

Motivation

The Athenz auth plugin obtains a role token from the ZTS server to be used for authentication. Depending on the user's environment, it may be necessary to access ZTS through a proxy, but currently the proxy URL is not configurable via the auth plugin.

Modifications

Modified the Athenz auth plugin to enable users to configure the proxy URL for ZTS. Proxy configuration is supported in v1.12.8 and later of the Athenz library.
AthenZ/athenz#2841

Verifying this change

• Make sure that the change passes the CI checks.

Does this pull request potentially affect one of the following parts:

If yes was chosen, please highlight the changes

• Dependencies (does it add or upgrade a dependency): (no)
• The public API: (no)
• The schema: (no)
• The default values of configurations: (no)
• The wire protocol: (no)

Documentation

• Does this pull request introduce a new feature? (yes)
• If yes, how is the feature documented? (not documented)
• If a feature is not applicable for documentation, explain why?
  • Once this PR is merged, I'm going to send a PR to https://github.com/apache/pulsar-site to update the documentation.