Share HTTP connectors between providers and clients (#2876)

## Motivation and Context
Clients using separate connectors is mostly confusing and troublesome
for customers.

## Description
Change the behavior of `ConfigLoader::http_connector` to set both the
client & credential provider HTTP connector.

**Note**: It is still possible to separate control clients for the
credential provider. Because `HttpConnector` is used, the timeout
settings can still be late-bound.

## Testing
Unit tests.

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the AWS
SDK, generated SDK code, or SDK runtime crates

----

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