Add Pipeline and Step Comments

[goodwillpunning]

Changes

This PR adds a comment attribute to the PipelineConfig and Step classes.

What does this PR do?

Adds the ability for pipeline authors to document what a pipeline and/or step does for future maintainers (including themselves).

Relevant implementation details

For pipelines containing many steps, it becomes unclear what effect a pipeline or step has. To discern what a step does, for example, a pipeline maintainer must look at the extract_source attribute, navigate to the Python or SQL file, and then read through the file to determine what a step does.

Caveats/things to watch out for when reviewing:

Linked issues

Resolves #1470.

Functionality

• added relevant user documentation
• added new CLI command
• modified existing command: databricks labs remorph ...
• Augments the PipelineConfig and Step classes with a metadata attribute.

Tests

• manually tested
• added unit tests
• added integration tests

[github-actions]

✅ 15/15 passed, 1 skipped, 10s total

_{Running from acceptance #534}

[goodwillpunning]

Closing in favor of #1557 (has a signed commit this time).