Improve user agent logic

[jackwotherspoon]

User agent currently is set during the first connection call. Subsequent calls have no way of updating the custom user agent. Aka if you create a connector and use it with many drivers or many custom user agents only the ones used on the first connect will be set.

Instead we should keep a set() of user agents and add as we go and update the client.

cloud-sql-python-connector/google/cloud/sql/connector/connector.py

Lines 215 to 220 in bb04427

	self._client = CloudSQLClient(
	self._sqladmin_api_endpoint,
	self._quota_project,
	self._credentials,
	user_agent=self._user_agent,
	driver=driver,