[identity] Add /inbound_keys HTTP handler

Summary:
Added HTTP endpoint for inbound keys. Request and response format described in ENG-8290.

Added some utility traits to leave handler with only business logic.

Depends on D12300, D12303

Test Plan:
Registered a user with a few devices.

• Called GetInboundKeysForUser RPC
• Called /inbound_keys HTTP endpoint

Compared results.

Reviewers: marcin, varun, will

Reviewed By: marcin, varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12304