Add MapType as JSON-compatible

[srowen]

Description of changes:

dataframe_to_mds does not directly support the Spark Map type as there is no direct analog in streaming (?)
However, the dataframe_to_mds will accept "unsupported" Spark types if they are JSON-compatible, and if the user specifies type 'json' for the column.

However the logic of is_json_compatible doesn't include MapType, when a map is conceptually fine as a dict, and Spark uses simple dicts to represent such maps in pandas DFs. As long as the map's key and value types are JSON compatible, it should be considered JSON compatible.

With this change, it's possible to have columns including a map type successfully convert to JSON in an MDS file if the given type for the column is specified as 'json'.

Merge Checklist:

General

• I have read the contributor guidelines
• This is a documentation change or typo fix. If so, skip the rest of this checklist.
• I certify that the changes I am introducing will be backward compatible, and I have discussed concerns about this, if any, with the MosaicML team.
• I have updated any necessary documentation, including README and API docs (if appropriate).

Tests

• I ran pre-commit on my change. (check out the pre-commit section of prerequisites)
• I have added tests that prove my fix is effective or that my feature works (if appropriate).
• I ran the tests locally to make sure it pass. (check out testing)
• I have added unit and/or integration tests as appropriate to ensure backward compatibility of the changes.

[snarayan21]

minor nits.

If possible can you add a simple unit test?

[srowen]

I think this is otherwise good to go - there are existing and one new test. I can add more too

[snarayan21]

minor nit: variable name change. otherwise lgtm

[srowen]

@snarayan21 said I could merge after that change, so here goes!