[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:

1. Emscripten does not handle int64_t
2. 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