feat: Target level SQLConnector instance shared with Sinks

[BuzzCutNorman]

Feature scope

Targets (data type handling, batching, SQL object generation, tests, etc.)

Description

Currently an SQL target will open and hold a session to the target for each stream it is given. This means if a tap has 100 streams the SQL target will open 100 session and hold them until the meltano run is finished. It would be nice if SQL targets could utilize a single connector and leverage the SQLAlchemy pool of connection it has.

This can be accomplished If an instance of the target's connector class is initialized and held as a property and that connector property is passed on to SQLSink instances when they are initialized by the add_sink method. The property code might look like this:

class SQLTarget(Target):
    """Target implementation for SQL destinations."""

    default_connector_class = SQLConnector
    _target_connector: SQLConnector = None

    @property
    def target_connector(self) -> SQLConnector:
        """The connector object.

        Returns:
            The connector object.
        """
        if self._target_connector is None:
            self._target_connector = self.default_connector_class(dict(self.config))
        return self._target_connector

The add_sink method would need to pass self.target_connector to the SQLSink when it is initialized. A couple of quick notes:

• The SQLSink class has a connector parameter while the base Sink class does not.
• The add_sink method is decorated with @final so it cannot be overloaded in the SQLTarget class.

The code could look something like this:

   @final
    def add_sink(

		... comments and code removed to show the change better
		
        sink = sink_class(
            target=self,
            stream_name=stream_name,
            schema=schema,
            key_properties=key_properties,
            connector=self.target_connector,
        )
		
		... some code removed to assist with clarity
		
        return sink

An example of this working in a target can be found in target-mssql--buzzcutnorman:target.py and target-postgress--buzzcutnorman:target.py