Support setting service endpoints via config/environment variable

[endgame]

Official SDKs support setting service endpoints via config settings as well as environment variables: https://docs.aws.amazon.com/sdkref/latest/guide/feature-ss-endpoints.html

We should do the same, eventually. (Low priority, since nobody has stumbled over the lack of this feature until very recently.)

[chreekat]

I used a version of the patch in #1013 in a new project today. :) So I may still be one of the first people who needed this, but now I have two use cases!

[chreekat]

I wrote #1046 to implement this, but I've got one question that could use some feedback (on the PR).