[identity] Add Serialize trait for inbound keys types

Summary:
Added serialize derive for structs that are part of the inbound keys response
This will be required for sending these in the inbound keys HTTP response

Alternatively, I could create a dedicated JSON-only structs and provide conversion traits, not to obfuscate existing structs, but this can be done at any time as a follow up.

Test Plan:
In get_inbound_keys handler, used serde_json::to_string() to print out results.
The JSON structure is as proposed in ENG-8290.

Reviewers: marcin

Reviewed By: marcin

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12300