Create a gRPC endpoint for active sessions

[jamesturner246]

This will be the simplest of the two endpoints, as it is only a placeholder.

Currently the backend tech for the active session database is unknown, so we should just mock our own active session name and actor.

In the drunc codebase, see how endpoins are organised and implemented in the drunc/src/{controller,process_manager} directories, and make the session_manager directory, following the same directory structure. These directories may contain some extra shell script stuff, but maybe we don't need in ours.

The endpoint should return a size one list of a structure containing name and actor for now.