[keyserver][lib] Move clientResponseInputValidator to lib

Summary:
Initially, I thought I needed to move these to lib because I was about to introduce a new endpoint that took ClientResponses as input.

I ended up removing ClientResponses from the input, so this diff isn't really necessary.

That said, I feel like it's probably "good practice" in our codebase to move all validators to lib, so I decided to put this diff up. If reviewers disagree, I'm open to abandoning it.

Note that this also required moving some validators for reports.

Depends on D12360

Test Plan: This is a pure move diff. Careful code inspection and Flow

Reviewers: tomek

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D12361