Allow downstream builds to self-identify for /v1/country, 404 /v1/geolocate

[alexcottner]

This PR is a combination of small changes, mostly from feedback around MLS migration:

1. Adds parameter for api keys file path
2. Allows custom keys to be provided to /v1/country endpoint (fixes Allow downstream firefox builds to self-select API keys to /country #151)
3. Adds unit tests around v1/country for keys
4. Removes route to /v1/geolocate, causing a 404 to be returned
5. Makes a small adjustment to utils.rs for an updated lint rule
6. Updates readme with notes on MLS endpoints

[leplatrem]

Apart from the silent behaviour when no keys are loaded, this should be ok

[alexcottner]

Apart from the silent behaviour when no keys are loaded, this should be ok

Going to work on that in a followup. Thinking maybe we should just load those into settings along with the env vars.