[keyserver] Replace public_key column with signed_identity_keys in cookies table

Summary: Remove public_key column from the cookies table and replace with signed_identity_keys which will hold primary + notification (ed25519, curve25519) public key pairs (stringified payload with signature).

Test Plan:
Migration:
Run the migration and see that DB is as expected:

Create DB:
Close reading, made sure to match social_proof line exactly.

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6907