[keyserver] Persist signedIdentityKeysBlob in keyserver on login

Summary:
On login (for new clients), we persist signedIdentityKeysBlob in the signed_identity_keys column of the cookies table.

Next up is handling SIWE on web. Then will handle the native client. Then will add some validation regex tests just to make things a bit more robust.

Test Plan:
Log in succeeds and signed_identity_keys is populated as expected when signedIdentityKeysBlob is included in login request:

Log in succeeds and signed_identity_keys is <null> when signedIdentityKeysBlob is NOT included in login request (EG old clients):

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6908