[keyserver] Modify registerRequestInputValidator to handle primaryIdentityPublicKey

Summary:
We're not doing anything with it yet on the keyserver side, just to prevent request from failing.

Depends on D6673

Test Plan: Will be tested implicitly by subsequent diffs.

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6674