Remove third party types from public APIs (#2845)

## Motivation and Context
Addresses item 1, 3, and 9 in #2413 

## Description
This PR removes the third party types as follows:
- removes `InvalidHeaderValue` from public API in `aws-http`
- removes `SendError` from public API in `aws-smithy-async`
- removes `xmlparser` from public API in `aws-smithy-xml`

Those types were exposed to public APIs primarily due to the
implementation of traits, e.g. `From` and `Iterator`. Those
implementations are used internally (such as XML deserialization and
converting an error type) so we should be able to hide them within
crates.

## Testing
- [x] Passed 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
- [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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Co-authored-by: ysaito1001 <awsaito@amazon.com>