[SQLite] add MessageToDevice entity
Summary:
ENG-6739.
Structs used to operate on SQLite DB.
Introducing ClientMessageToDevice to make it work gracefully on both web and native:
- Emscripten does not handle int64_t
- On native - we pass timestamps as strings anyway
Depends on D11168
Test Plan: Build WASM - functionality tested later in the stack.
Reviewers: marcin, michal
Reviewed By: marcin
Subscribers: ashoat, tomek
Differential Revision: https://phab.comm.dev/D11169