Add support for MAP type to SQLAlchemy dialect

[hashhar]

Description

Adds support for MAP type to SQLA dialect.
Fixes #397
Supersedes #398

This preserves commit authorship from #398.

Release notes

(x) Release notes are required, with the following suggested text:

* Add support for `MAP` to SQLAlchemy dialect.

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[hashhar]

For what it's worth this may not be complete juding by the JSON impl at ab0e596 and the STRUCT impl in BigQuery at https://github.com/googleapis/python-bigquery-sqlalchemy/blob/main/sqlalchemy_bigquery/_struct.py

Not all operations may work on the type and are out of scope for this pull-request.
Trying to even get this to work (just being able to read and write values correctly) took too much effort.

I had planned to add support for all structural types (ARRAY, MAP and ROW) but I'll leave that to folks who have strong motivation to work with the complex SQLAlchemy API. 😩