Move `Message`, `Header`, `HeaderValue`, and `StrBytes` from...

Move `Message`, `Header`, `HeaderValue`, and `StrBytes` from `aws-smithy-eventstream` to `aws-smithy-types` (#3139)

## Motivation and Context
Fixes the following statement in
https://github.com/awslabs/smithy-rs/pull/3114

> A notable breaking change is the error type of the new recv method.
Previously, it was [SdkError<E,
RawMessage>>](https://docs.rs/aws-smithy-http/0.57.0/aws_smithy_http/event_stream/struct.Receiver.html#method.recv)
but it is now a BoxError. This means that upon an event receive error,
customers can only show what the error is, but not match on it and take
some actions.

so that the `recv` method on a new-type wrapper
`aws_smithy_http::event_stream::Receiver` can expose `SdkError<E,
RawMessage>>` in public API. It is the responsibility of the above PR to
move
[RawMessage](https://github.com/awslabs/smithy-rs/blob/c7f0d5dc33937d65841d74b5b828cc77ed1d2db4/rust-runtime/aws-smithy-http/src/event_stream/receiver.rs#L92-L98)
from `aws-smithy-http` to `aws-smithy-types` but doing so first requires
`Message` to be moved to `aws-smithy-types`, hence this PR.


## Description
Moving `Message` to `aws-smithy-types` also requires moving the types it
depends upon, namely `Header`, `HederValue`, and `StrBytes`. However,
their serialization/deserialization methods (`read_from` and `write_to`)
remain in `aws-smithy-eventstream` and they are converted to free
functions, with the aim of moving things as minimally as possible.

`aws-smithy-eventstream` has fuzz testing. It previously implemented the
`Arbitrary` trait for moved types, but since they are now in
`aws-smithy-types`, we need to have newtypes for them to enable the
trait due to the orphan rules. A newly added module
`aws-smithy-eventstream::arbitrary` is exactly for that purpose.

## Testing
Relied on the existing 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

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