[identity] use ed25519 key instead of curve as map key

Summary: we should be using the ed25519 key as the map key in DDB to be consistent with comments in the .proto file and constants.rs. this was an oversight on my part

Test Plan: checked that the DDB table map was now keyed on the correct signing public key

Reviewers: jon, bartek

Reviewed By: jon

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7595