[keyserver] Verify signedIdentityKeysBlob signature in logInResponder

Summary:
Pass primaryIdentityPublicKeys.ed25519, signedIdentityKeysBlob.payload and signedIdentityKeysBlob.signature to olmUtil.ed25519_verify(...) to ensure that the signature is valid.

Depends on D6896

Test Plan: I tested with a valid signature and observed that ed25519_verify(...) succeeded. I tested with an invalid signature ("blah"), and observed that olmUtil.ed25519(...) threw an exception.

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6897