[identity] Refactor device list types

Summary:
Did some refactoring to make it clear what some structs are designed to be. Previous structure was designed to have Identity sign device lists as well, but we finally opted out of it (only primary device signs them).

Short summary:

• UpdateDeviceListRequest is a request coming from primary device calling update RPCs
• SignedDeviceList is a JSON payload, Whitepaper 6.2.3
• RawDeviceList is a helper struct for inner stringified JSON (see whitepaper)
• DeviceListUpdate is actual payload containing device list data to be processed in DDB transaction (some more details in the next diff)

Depends on D11897

Test Plan: Unit and integration tests. Device list logic works as before.

Reviewers: kamil, will

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11898