feat(sdk): autogenerate urn types

[hsheth2]

Also fixes an issue where the models documentation wasn't showing up properly with sphinx.

Remaining items:

• Make this work with custom packages
• Add a note to about updating datahub
• Change the canonical import path to datahub.metadata.urns

Checklist

• The PR conforms to DataHub's Contributing Guideline (particularly Commit Message Format)
• Links to related issues (if applicable)
• Tests for the changes have been added/updated (if applicable)
• Docs related to the changes have been added/updated (if applicable). If a new feature has been added a Usage Guide has been added for the same.
• For any breaking change/potential downtime/deprecation/big changes an entry has been made in Updating DataHub

[asikowitz]

Some doctests / examples of what this is doing would be nice. I eventually got it but took a sec. Also kinda odd to me that it basically ignores parentheses, besides checking that there's the right amount of them in an appropriate order.

It's less efficient but think it'd be clearer to break this up:

PARENS = ["(", ")"]
def _split_entity_id(entity_id: str) -> List[str]:
    if not (entity_id.startswith("(") and entity_id.endswith(")")):
        return [entity_id]
    parens = [v for v in entity_id if v == "(" or v == ")"]
    if not _parens_valid(parens):
        raise InvalidUrnError(...)
    return entity_id[1:-1].split(",")

I think that's what this function is doing?

[hsheth2]

this was copied from the existing urn code, so I'm inclined to leave it as is since it seems to work

[asikowitz]

Should we use the constants above here? Or that's just overkill

[hsheth2]

I think it's fine