Move default endpoint resolver into a runtime plugin (#3072)

## Motivation and Context
In preparation for inlining the old endpoint traits, I'm cleaning up the
endpoint resolver behavior.

## Description

1. The `Config` field of `service_runtime_plugin` was unused to I
deleted it
2. Store the endpoint resolver in `RuntimeComponents` instead of in
operation config. This allows a lot of complex logic around
`ConfigOverride` to be removed.
3. Move the default endpoint resolver into a runtime plugin.
4. Don't have a fallback implementation for `EndpointResolver`, allow
final construction of `RuntimeComponents` to fail
5. `ServiceRuntimePlugin` has been changed to `Defaults` so that it
doesn't overwrite settings set by the service config


## Testing
Well covered by existing UT / IT
## 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._