Add endpoint resolver to SdkConfig (#1300)

* Add endpoint resolver to SdkConfig

This commit adds an `endpoint_resolver` field to SdkConfig, which flows
through to the `endpoint_resolver` field on each individual service
configuration. To avoid a circular dependency between the aws-types
crate and the aws-endpoint crate, the `AwsEndpoint` type and its
dependencies are moved from the aws-endpoint crate to the aws-types
crate, then rexported in aws-endpoint.

Fix aws-labs/aws-sdk-rust#396.

* impl From<'static str> for signing service and signing region

* Code generate for the static string implementation

* Add EndpointResolver to aws_config

* Add docs and adapt test

* Fix test

* Fix endpoint config customizations test to remove dead code

* Re-add `.set_endpoint`

* Remove print and update changelog

Co-authored-by: Russell Cohen <rcoh@amazon.com>