[refactor][java-client] Reduce code duplication in GET requests in admin client

[andrasbeni]

Motivation

This is a continuation of #16377 to reduce code repetition in the admin client.

Modifications

Move the functionality implemented in InvocationCallbacks to a common place.
Hide creation of CompletableFuture to be consistent with other HTTP method
implementations (e.g. asyncPutRequest).

Verifying this change

• Make sure that the change passes the CI checks.

This change is a trivial rework / code cleanup without any test coverage.

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)
• The rest endpoints: (no)
• The admin cli options: (no)
• Anything that affects deployment: (no)

Documentation

• doc-not-needed
  This is pure refactoring, no behavior is changed.

[github-actions]

@andrasbeni Please provide a correct documentation label for your PR.
Instructions see Pulsar Documentation Label Guide.

[andrasbeni]

/pulsarbot rerun-failure-checks

[andrasbeni]

/pulsarbot rerun-failure-checks

[andrasbeni]

/pulsarbot rerun-failure-checks

[andrasbeni]

/pulsarbot rerun-failure-checks

[andrasbeni]

/pulsarbot run-failure-checks