Consider AWS_ENDPOINT_URL configuration when resolving service endpoints

[whummer]

This PR adds minor enhancements to have proper handling of the AWS_ENDPOINT_URL configuration. Previously we were only using it for the get_service_endpoint(..) function, but not for get_service_endpoints(..). This was leading to issues when using the

Summary of changes:

• move logic around AWS_ENDPOINT_URL into get_service_endpoints(..)
• call get_service_endpoints(..) from get_service_endpoint(..), and select the corresponding service entry from the dict. There is a tiny performance penalty in this approach, which seems acceptable given that it allows us to keep the endpoint construction logic in a central place. (this now also works transitively for get_service_ports(..))
• update docs to cover $AWS_ENDPOINT_URL configuration

Side note: Overall, this library (esp. the host/port handling bit) should soon become obsolete (there's already a draft PR to remove it from LocalStack core, and we should eliminate it from awslocal soon as well).

[simonrw]

There are no tests to cover this functionality - does it make sense to add tests/a test to check the behaviour of the environments variables (AWS_ENDPOINT_URL, LOCALSTACK_HOST, USE_SSL) to make sure they behave correctly?

[simonrw]

There are no tests to cover this functionality - does it make sense to add tests/a test to check the behaviour of the environments variables (AWS_ENDPOINT_URL, LOCALSTACK_HOST, USE_SSL) to make sure they behave correctly?

nvm it's not worth it since as you say

Overall, this library (esp. the host/port handling bit) should soon become obsolete (there's already a draft PR to remove it from LocalStack core, and we should eliminate it from awslocal soon as well).

[simonrw]

Works as expected, LGTM! 🎉