[terraform] add FCM secrets config

Summary: This will be added to AWS KMS secret manager as JSON and is exposed as an environment variable in the container.

Test Plan: yarn init-local-cloud - the exact I'll test exact functionality on staging after entire stack will be accepted.

Reviewers: bartek, marcin

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12751