Expose Body::streaming as public API

[Xuanwo]

Hi, http_body is reaching 1.0 now, maybe it's time for us to make Body::streaming public?

reqwest/src/async_impl/body.rs

Lines 125 to 142 in 0720159

	// pub?
	pub(crate) fn streaming<B>(inner: B) -> Body
	where
	B: HttpBody + Send + Sync + 'static,
	B::Data: Into<Bytes>,
	B::Error: Into<Box<dyn std::error::Error + Send + Sync>>,
	{
	use http_body_util::BodyExt;
	let boxed = inner
	.map_frame(|f| f.map_data(Into::into))
	.map_err(Into::into)
	.boxed();
	Body {
	inner: Inner::Streaming(boxed),
	}
	}

This change will:

• Close Implementation of http_body::Body for reqwest::Body #1261
• Allow users to pass non-continuous bytes as body without converting them into a stream.
• Allow wasm users to pass iter/stream without collect them all (wasm client doesn't expose wrap_stream)

I'm willing to submit a PR if you feel this looks good.

[rogusdev]

Would this make it possible to also easily add streaming in wasm per #2248 ?