feat(StateMediator): add stop events children to S3 mediator

[runkelcorey]

Summary of changes

Asana Ticket: 📸↩︎ fetch s3 file (StateMediator)

1. Adds stop_event_children using S3Mediator to StateMediator following the model of cr_crowding_children
2. Updates logging to account for stop events failures
3. Updates config to look for stop events env vars
4. Updates README to explain why the app won't start locally

This approach uses a feature flag set to false by default so that this PR is release-ready. Additionally, the functions to check for the env vars won't raise an error if the variables are not found. Eventually, they probably should. First, I want to merge updates to the ECS configuration and IAM policies so that the API can actually find variables and read the stop_events feed.

How were these changes validated?

1. Ran mix phx.server locally, saw success when STOP_EVENTS_ENABLED is false and when all stop events env vars were configured

What should reviewers consider?

1. Does S3Mediator deserve an integration test? I feel like we should test the S3Mediator default/test paths but idk—it works now and is tested in dev envs. I guess we could mock the S3Filesystem?
2. Should I consolidate the StateMediator children now or after an eventual prod release of stop_events? It isn't unwieldy but it's starting to look crufty

[lemald]

One quick question on the README changes, but overall LGTM. 👍