[terraform] add Web Push 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
Reviewed By: bartek
Subscribers: ashoat, tomek
Differential Revision: https://phab.comm.dev/D12840