Deploy-time configuration

[mfisher87]

Issue #421 got me thinking about this a little bit. It's a common use case to want to deploy the same app in multiple environments, but in one environment, talk to UAT or SIT. I think we should support this by allowing users to set an envvar, e.g. EARTHACCESS_EDL_ENV or EARTHACCESS_EDL_HOSTNAME that will override earthaccess' selected earthdata AUTH_HOST (currently always production unless you monkeypatch, but will be configurable after a new feature @danielfromearth is working on)

earthaccess/earthaccess/auth.py

Line 24 in f476685

AUTH_HOST = "urs.earthdata.nasa.gov"

Are there other things about earthaccess that we would want to configure/override at deploy-time?