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.

https://github.com/GoogleCloudPlatform/cloud-sql-python-connector/blob/bb044277e58475b3c1dfc2a497935aa97c8f2e4d/google/cloud/sql/connector/connector.py#L215-L220