Add `secure` parameter to Connectors to simplify TLS.

[jwp]

With the deprecation of ssl.wrap_socket, carrying TLS arguments makes little sense and is inappropriately limiting.

Provide an alternative to the ssl* parameters (excluding sslmode) with a single secure parameter. Where secure will provide the entire functionality required by python.socket.SocketFactory.secure. Users will pass the wrap_socket method of the SSLContext they (already) configured as the secure keyword.

Possible separate patch: make this usable with postgresql.open indicators, by adding a security dictionary to postgresql.sys where applications may configure the set of secure functions used. postgresql.open("pq://user@host/database?[security]=tls-context-name")?

[jwp]

Allow postgresql.open to select the security context using the host and port as the key in the sys.security dictionary?