[keyserver] add endpoint to acknowledge policy

Summary:
Endpoint which allows acknowledging policy, which is specified in request.

Preferring HTTP connection because in the future we maybe will want to close socket connection when policy is not accepted.

Context in ENG-2352

Test Plan:

1. hit endpoint with corrent and incorrect policy to check if validator works
2. check if value is properly updated in policy_acknowledgments table

Reviewers: tomek, atul

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D5821