Duplicate schema info classes

[pmantica1]

We added the SQLSchemaInfo class without deleting the SQLAlchemySchemaInfo class and they are are almost exact duplicates. We also added the GenericSchemaInfo class without deleting the CommonSchemaInfo class.

[pmantica1]

The SQLAlchemySchemaInfo and SQLSchemaInfo are almost identical externally except for the constructor. The SQLAlchemySchemaInfo takes in a schema and type_equivalence_hints and the SQLSchemaInfo takes in a generic_schema_info. If we are shifting toward using generic_schema_info as input, then we might also want to change the function signatures of _create_sql_schema_info, get_sqlalchemy_schema_info and make_sqlalchemy_schema_info. (The fact that we have so many different constructors is a different issue though).