[FEAT] Allow user provided schema and schema inference length for read_sql

[colin-ho]

Enables users to provide a schema to read_sql, as well as an infer_schema flag to turn on/off inference, and infer_schema_length to customize the length of the scan for inference.

Note that infer_schema_length only applies when SQLAlchemy is used to read, otherwise ConnectorX is able to provide the schema with a limit=0 as it is an arrow backed driver. This is a new optimization.

This PR also has a few refactors, namely moving the construct_sql_query and the read_schema logic to the SQLConnection object instead of the ScanOperator. This is so that schema inference also has context on the driver that will be used.