Clean up re-exports and improve 'BuildError' conversions (#3173)

## Motivation and Context
- Fixes #3171 
- Fixes #3155 
- Fixes #3172 


## Description
- Add `configReExport` to make it easy and consistent to re-export types
into the config module when they are used!
- Add this for a bunch of types and clean up some (now) dead code
- Re export `BuildError`
- Enable converting from `BuildError` into `SdkError`

There are probably more places this can be used, but I'd like to keep
this change small to facilitate easy backport to 0.57.

## Testing
CI

## 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

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