[keyserver] Introduce validateOutput function

Summary:
Introduces validateOutput function that validates and (right now blocked with a flag) converts the ids of the responses. Also introduces a pair of functions convertServerIDsToClientIDs (used in validateOutput and in tests) and convertClientIDsToServerIDs (only used in tests right now, will be used in the future diff for the input conversion).

Depends on D7710

Test Plan:

• yarn jest
• Test if the app works fine after adding validateOutput to responders in the later diffs

Reviewers: kamil, tomek

Reviewed By: tomek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D7711