Add a simple pulsar source connector.

[cckellogg]

What is the purpose of the change

This pull request adds a pulsar source connector which will enable flink jobs to process messages from pulsar topics.

Brief change log

• Add a PulsarConsumerSource connector

Verifying this change

This change adds unit test to verify checkpointing and batch message acknowledgements.

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? JavaDocs

[sijie]

overall looks good!

[sijie]

I think flink is using tab for indents if I remembered directly. it might be worth double-checking before you send the pull request.

[sijie]

missing apache header