Port fix for debug bound on ResolveEndpoint (#2636)

Port #2630 to the `main`
branch.

Adding `Debug` bound on `ResolveEndpoint` in
#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.

- [ ] Passed tests in CI

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._

Co-authored-by: Yuki Saito <[email protected]>

Author: ysaito1001