[improve][test] Add integration test for additional servlets

[andrasbeni]

Master Issue: #17132

Motivation

Increase test coverage of additional servlet functionality.

Modifications

• Created an additional servlet RandomAdditionalServlet.
• Created a test that configures the additional servlet for the brokers and the proxy and verifies that the servlet functions correctly

Verifying this change

• Make sure that the change passes the CI checks.

This change added tests and can be verified as follows:

Added integration tests for additional Servlets

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 change only affects tests

[andrasbeni]

/pulsarbot rerun-failure-checks

[andrasbeni]

/pulsarbot rerun-failure-checks