[FLINK-11853] [rest] Minimal implementation of POST for /jars/:jarid/plan

[stephenc]

What is the purpose of the change

Some HTTP Client libraries enforce that GET requests cannot have a request body which means that the request body form of the existing GET /jars/:jarid/plan cannot be used with those clients.
This change replicates the end-point for POST requests also so that the functionality can be accessed irrespective of the chosen HTTP client library.

Brief change log

• Add the new endpoint.

Verifying this change

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

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

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? yes
• If yes, how is the feature documented? The automatic REST API documentation generator documents the additional API endpoint

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Review Progress

• ✅ 1. The [description] looks good.
  • Approved by @zentol [PMC]
• ✅ 2. There is [consensus] that the contribution should go into to Flink.
  • Approved by @zentol [PMC]
• ❓ 3. Needs [attention] from.
• ✅ 4. The change fits into the overall [architecture].
  • Approved by @zentol [PMC]
• ✅ 5. Overall code [quality] is good.
  • Approved by @zentol [PMC]

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[zentol]

Please regenerate the REST API documentation by running mvn package -Dgenerate-rest-docs -pl flink-runtime-web,flink-docs -am -nsu -DskipTests

Beyond that this looks good.

[zentol]

@flinkbot approve-until quality