[Tunnelbroker] Define undelivered-messages table in rust

Summary:
Describe the table in rust.
Part of: https://linear.app/comm/issue/ENG-3822

Test Plan: N/A, this is able design

Reviewers: varun, bartek, ashoat

Reviewed By: varun, ashoat

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D7720