gio: Add GTlsInteraction interaction method invocation guarantees

gio: Add GTlsInteraction interaction method invocation guarantees

 * Add 'invoke' style method, which can be used to call an interaction
   from any thread. The interaction will be run in the appropriate
   #GMainContext
 * Sync methods can be called whether main loop is running or not.
 * Derived classes can choose to implement only sync or async
   interaction method, and the invoke method will fill in the blanks.
 * Documentation for the above.
 * Tests for the above.

https://bugzilla.gnome.org/show_bug.cgi?id=657567