[identity] Extract function to ed25519 verify message

Summary: This code is used in two places, and is going to be used in third one, so extracted this to a separate func.

Test Plan:

• Checked that nonces are still verified correctly (also Commtest proves it)
• Set the KEYSERVER_PUBLIC_KEY env variable and checked that reserved usernames RPCs pass verification

Reviewers: will, kamil

Reviewed By: will

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11957