Make cert validation callback's attachSSLContext method be const
Make cert validation callback's attachSSLContext method be const

Summary: Makes the method const to make the API enforce no side effects as a result of attempting to recreate contexts.

Reviewed By: mingtaoy

Differential Revision: D19156166

fbshipit-source-id: 09089cdd62ac9d53e83f8dc1d5e1e2271182ac98

facebook/wangle,facebook/wangle,facebook/wangle