Port fix for debug bound on ResolveEndpoint (#2636)

## Motivation and Context
Port https://github.com/awslabs/smithy-rs/pull/2630 to the `main`
branch.

## Description
Adding `Debug` bound on `ResolveEndpoint` in
https://github.com/awslabs/smithy-rs/pull/2577

 was a breaking change.
Unfortunately, the semvar check that ran in that PR barked at a
different thing (i.e. barked at moving `StaticUriEndpointResolver` and
`DefaultEndpointResolver` from `aws-smithy-runtime-api` to
`aws-smithy-runtime`). Eventually the breaking change got merged to the
`main` branch.

This PR reverts the change in question and implements the `Debug` trait
for `ResolveEndpoint` in a semvar non-breaking way.

## Testing
- [ ] Passed tests in CI

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Co-authored-by: Yuki Saito <awsaito@amazon.com>