[FLINK-7855] Port JobVertexAccumulatorsHandler to REST endpoint

[FangYongs]

What is the purpose of the change

Port JobVertexAccumulatorsHandler to REST endpoint

Brief change log

• Add JobVertexAccumulatorsHandler for DIspatcherRestEndpoint
• Add JobVertexAccumulatorsHeaders/JobVertexAccumulatorsInfo for rest response
• Add JobVertexAccumulatorsInfoTest

Verifying this change

This change added tests and can be verified as follows:

• Added test case JobVertexAccumulatorsInfoTest to generate the json response

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)

Documentation

• Does this pull request introduce a new feature? (no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

[tillrohrmann]

Thanks for your contribution @zjureel. Changes look good to me. Merging this PR.