fix(sqlalchemy-bigquery): Support Creation of JSON columns

[waltaskew]

Fixes #15761

[gemini-code-assist]

Code Review

This pull request adds support for the JSON data type in the BigQuery SQLAlchemy dialect by implementing the visit_JSON method and adding a corresponding unit test. Feedback was provided to include a JSONB alias for better compatibility with models using PostgreSQL-specific types.

[gemini-code-assist]

Consider adding an alias for JSONB to improve compatibility with models that use the JSONB type (e.g., those originally designed for PostgreSQL). Since BigQuery's JSON type is binary-optimized, it is the appropriate mapping for JSONB in this dialect.

    def visit_JSON(self, type_, **kw):
        return "JSON"

    visit_JSONB = visit_JSON