[FLINK-28654] Include JMX metrics reporter plugin

[dylanmei]

What is the purpose of the change

Include JMX metrics reporter plugin to enable local developer to view metrics with simple console tools. JMX also facilitates integration with collectors such as Jolokia and New Relic Java agent.

Brief change log

Include JMX metrics reporter as an available plugin for the operator container image.

Verifying this change

(Please pick either of the following options)

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

After building Docker image, plugin is verified on container file system

$ docker run --rm -it flink-operator ls -lah /opt/flink/plugins/flink-metrics-jmx/
total 36K
drwxr-xr-x 1 flink flink 4.0K Jul 25 13:42 .
drwxr-xr-x 1 flink flink 4.0K Jul 25 13:42 ..
-rw-r--r-- 1 flink flink  18K Jul 24 16:21 flink-metrics-jmx-1.15.1.jar

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

• Dependencies (does it add or upgrade a dependency): yes
• The public API, i.e., is any changes to the CustomResourceDescriptors: no
• Core observer or reconciler logic that is regularly executed: no

Documentation

• Does this pull request introduce a new feature? yes
• If yes, how is the feature documented? not documented

[gyfora]

Looks good! Thank you for the contribution :)