KAFKA-13731: Allow standalone workers to be started without providing any connector configurations

[C0urante]

Jira

(Copied from Jira):

In order to start a standalone Connect worker, it's currently necessary to provide at least two command-line arguments:

• The path to the worker config file
• One or more paths to a connector config file

This may be due to the now-inaccurate belief that standalone workers do not support the Connect REST API, which can be used to create/reconfigure/delete connectors at runtime. However, standalone mode does in fact expose the Connect REST API, and since that allows for connectors to be created after bringing up the worker, it is unnecessary to force users to supply at least one config for a connector to be run on the worker at startup time.

These changes remove that limitation from standalone mode, and update the Connect docs to be explicit about support for the REST API when running standalone.

No tests are added as the functional changes are trivial. I did perform an extremely rudimentary sanity check to ensure that, with these changes, a standalone worker could be brought up with just a worker config file and no connector configs, and a file sink connector could be created on that worker via the REST API after it finished startup.

Committer Checklist (excluded from commit message)

• [ ] Verify design and implementation
• [ ] Verify test coverage and CI build status
• [ ] Verify documentation (including upgrade notes)

[C0urante]

@showuon do you have time for this quick change?

[C0urante]

Thanks Mickael!