[services] Tunnelbroker - Database entities for messages.

Summary:
DynamoDB database entities for persisting messages that pass in tunnelbroker.

Related linear task: ENG-670

Dependencies:

• Depends on D3116 to provide ConfigManager.

Test Plan: Using entities for CRUD operations.

Reviewers: palys-swm, karol-bisztyga, ashoat, jimpo

Reviewed By: palys-swm, karol-bisztyga, ashoat, jimpo

Subscribers: benschac, ashoat, palys-swm, Adrian, atul, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3114