Add UNKNOWN lane mark type to map schema

[wqi]

PR Summary

This PR adds an UNKNOWN lane mark type to the map schema.

This change resolves an issue where attempting to load a static map containing such a lane mark type would result in a failure and error being raised.

Testing

Tested by loading a scenario containing an UNKNOWN lane mark type.

In order to ensure this PR works as intended, it is:

• unit tested.
• other or not applicable (additional detail/rationale required)

Compliance with Standards

As the author, I certify that this PR conforms to the following standards:

• Code changes conform to PEP8 and docstrings conform to the Google Python style guide.
• A well-written summary explains what was done and why it was done.
• The PR is adequately tested and the testing details and links to external results are included.

[benjaminrwilson]

Do you think we should add @unique to the other Enums in the changed file?

LGTM 👍