[Feature] Add docs to column info in manifest.json

[aranke]

Is this your first time submitting a feature request?

• I have read the expectations for open source contributors
• I have searched the existing issues, and I could not find an existing issue for this feature
• I am requesting a straightforward extension of existing dbt functionality, rather than a Big Idea better suited to a discussion

Describe the feature

column info should include doc block references like so:

"columns": {
  "my_column": {
    "name": "my_column"
    "description": "Hello, world!",
    "doc_blocks": ["doc.project_name.block_name", ...],
    ...
  }
}

Describe alternatives you've considered

N/A

Who will this benefit?

Anyone who wants to see what doc blocks are referenced in column descriptions, including the metadata team at dbt Labs.

Are you interested in contributing this feature?

Yes

Anything else?

Counterpart issue for node descriptions is here: #11000