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.

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