Need specific contexts for ssl-connections

[tvdijen]

It would be great to be able to control the context passed to stream_socket_client() on a per-connection basis. Especially for LDAP-authsources, it may be necessary to control the CAfile- and SNI-settings in the context.

[tvdijen]

Authsources, which are probably the primary use-case here, where addressed in #6.
Could use another PR for the Configuration TestSuite, perhaps one for the Metadata TestSuite, eventhough we're not checking metadata endpoints for their certificates yet.