Move `config_bag` and `type_erasure` to `aws-smithy-types` (#2735)

## Motivation and Context
This PR moves types and traits in those modules to the
`aws-smithy-types` crate.

## Description
This is to prepare for the upcoming refactoring where `SdkConfig` and
service configs (and both of their builders) will be backed by
`ConfigBag`.

## Testing
- [x] Pass tests in 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~~

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