feat(diagnostic): implement diagnostics class with gc invocation operation

[andrewazores]

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Related to #133
Depends on cryostatio/cryostat-core#459
NOTE Since this depends on an addition to -core, which publishes snapshot builds but for which the CI to do so is currently broken, this cannot be merged until that CI failure is resolved. Otherwise, Cryostat (and the associated upcoming Agent PR) will fail to build and all CI will be failing.

Description of the change:

Implements the Agent HTTP delegation side of the MBean operation invocation request, and adds a beta API endpoint which invokes the GC operation on the Agent HTTP or JMX connection to the specified Target JVM.

Motivation for the change:

Combined with the -core change and upcoming -agent and -web changes, this will allow the user to click a button in the Cryostat UI to send a request to their Target JVM requesting that it perform a garbage collection.

How to manually test:

1. Run CRYOSTAT_IMAGE=quay.io... bash smoketest.bash...
2. ...

[github-actions]

This PR/issue depends on:

• feat(diagnostic): add interface method for invoking MBean operations cryostat-core#459
  By Dependent Issues (🤖). Happy coding!