Make aws_smithy_http::byte_stream::ByteStream not expose SdkBody

[david-perez]

As mentioned here #1023 (comment)

We have decided that we will use aws_smithy_http::byte_stream::ByteStream,
making all methods referencing SdkBody #[doc(hidden)] (new, into_inner,
From<SdkBody> impl), and we will add generic methods that don’t reference
SdkBody.

This change is to allow server users to return ByteStreams in their output shapes, for streaming responses. The server so far has not used SdkBody, with all its code working against http_body::Body.

[david-perez]

There's some prior work here, making ByteStream and FsBuilder generic over the body type, but FsBuilder::build has in its implementation:

Ok(ByteStream::new(SdkBody::retryable(body_loader)))

And it wasn't rewritten successfully with the same semantics with an arbitrary body B.

[david-perez]

We also want ByteStream to be constructible from an arbitrary Stream, which might be an easier thing to do first.