Add Retrieval metrics POST endpoint

[jcace]

Add an endpoint to estuary-metrics to accept Retrieval stats (event) data.

Data will come from autoretrieve, using a Lassie Event Publisher, and it will be in the format described here

[jcace]

It looks like there is already a push endpoint defined: https://github.com/application-research/estuary-metrics/blob/main/rest/api/reporting-api/push.go

Perhaps it could be added here.

[jcace]

• add basic pre shared key authentication to the endpoint (environment variable) + simple middleware to verify