feat: add JSON type, bindparam support

[r1b]

Extracted from #1146, this is a smaller set of changes that establish a JSON type.

Changes:

• Export a JSON type
• Accept json_serializer / json_deserializer dialect kwargs
• Handle JSON bind parameters
• Tests

Once this is merged, users will be able to:

• Define tables with JSON columns
• Reference JSON columns in expressions
• Submit JSON query parameters (as STRINGs, with output conversion back to JSON)

In future PRs, we can add support for:

• JSON comparator conversion functions (BOOL / LAX_BOOL, etc.)
• JSON indexing operations

Fixes #399 🦕

[r1b]

note An alternative here would be to just preserve the STRING type and let BigQuery handle the cast, but this seems hard to reason about. It feels right to have the expression type in BigQuery match the expression type here in SQLAlchemy.

[r1b]

Looking at [1] and pondering this - it wouldn't be that surprising if a user wanted to submit invalid JSON as a query parameter at some point. This could be supported by parameterizing this behavior in the type, i.e: JSON(strict=False) or similar. Not worried about it right now, though.

[1] https://cloud.google.com/bigquery/docs/reference/standard-sql/json_functions#non-validation_of_string_inputs

[r1b]

note See https://docs.sqlalchemy.org/en/20/core/type_basics.html#sqlalchemy.types.JSON section "Customizing the JSON Serializer"