[services][feature-flags] Serve application/json instead of text/plain

Summary:
It is a more maintainable solution.

Depends on D6886

Test Plan: Send GET using curl and check if the response looks correct.

Reviewers: kamil, bartek, jon, varun

Reviewed By: bartek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D6947