[Connection Manager] Create API skeleton

This is a basic API for the connection manager, very similar to what exists in
connection_manager.h right now. The implementation and remaining features (e.g.
targeted announcements) will be filled in future CLs.

Bug: 272572974
Test: compiles
Change-Id: Ib63878b08b5c91b12e066d5f39db1c0489582750