Respect service-specific endpoint env variables

[chreekat]

Based on #1013. This version is enabled by default, and has fixed one bug in the original patch.

TODO:

• 1. What to do if the environment variable value is not a valid URL? The original patch silently skips it, and I'm still doing that here. But that is surprising behavior. Better would be... a warning? An error? I could use guidance on that front.
• 2. Add a mention of the default overrides somewhere??
• 3. Rename and expose customEndpoints from the module
• 4. Changelog should mention this PR number
• Reference Support setting service endpoints via config/environment variable #980 in the commit message

Opening as a draft so I can get feedback on #​1.

[chreekat]

Alas, this patch doesn't work the way it should.

I'm just using awscli for comparison. With awscli, if I just set AWS_ENDPOINT_URL, it isn't used as an absolute override for services. The service-specific endpoint prefixes are still applied. This patch doesn't do that yet. It should.

[chreekat]

core's Amazonka.Endpoint.defaultEndpoint has a lot of special-casing. Somebody with more knowledge would need to figure out how AWS_ENDPOINT_URL interacts with all of that.