[keyserver] Include siwePrimaryIdentityPublicKey in landingResponder response

Summary:
Include siwePrimaryIdentityPublicKey in landingResponder the same way we included siweNonce.

In subsequent diff the Context will be updated to consume (and make available to SIWE component) the value passed in here.

Depends on D6125

Test Plan: Will be tested implicitly by subsequent diffs

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6126