[keyserver] Introduce setCookieSignedIdentityKeysBlob

Summary:
Introduce setCookieSignedIdentityKeysBlob to update signed_identity_keys column of row in cookies table with id=cookieID.

This is to update serverDB when client SIGNED_IDENTITY_KEYS_BLOB response is received.

Depends on D7048

Test Plan: Ran query of that form manually in query console and it worked as expected. Will be tested implicitly by subsequent diffs in this stack.

Reviewers: ashoat

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D7049