[keyserver] Modify logInRequestInputValidator to handle primaryIdentityPublicKey

Summary: Context: https://linear.app/comm/issue/ENG-1967/update-clients-to-generate-a-publicprivate-keypair-when-authing

Test Plan:
Current "traditional" login flow continues to work as expected on native and web.

Request which includes primaryIdentityPublicKey will be tested implicitly by subsequent diffs.

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6656