Feature-gate public use of `http-body` and `hyper` within `aws-smithy-types` (#3088)

## Motivation and Context
Implements https://github.com/awslabs/smithy-rs/issues/3033

## Description
This PR hides behind cargo features the third-party types from
`http-body` and `hyper` crates that are used in`aws-smithy-types`'
public API. Customers need to opt-in by enabling a cargo feature
`http-body-0-4-x` in `aws-smithy-types` to create an `SdkBody` or
`ByteStream` using those third-party types. For more details, please see
[the upgrade
guide](https://github.com/awslabs/smithy-rs/discussions/3089

).

As can been seen from code changes, to reduce the surface area where we
need to feature-gate things, we have fused the
`aws_smithy_types::body::Inner::Streaming` enum variant into
`aws_smithy_types::body::Inner::Dyn` variant, thereby removing
`SdkBody::from_dyn`.

## Testing
Relied on existing tests in CI

## Checklist
- [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: Zelda Hessler <zhessler@amazon.com>