Incorrect `RequestRejection` yielded when `@httpPayload` targets string shape

[david-perez]

And the incoming value is not valid UTF-8. Indeed, consider:

@http(uri: "/http-payload-on-string", method: "POST")
operation HttpPayloadOnStringOperation {
    input: HttpPayloadOnString
}

structure HttpPayloadOnString {
    @httpPayload
    message: String
}

We generate:

pub(crate) fn de_message_payload(
    body: &[u8],
) -> std::result::Result<
    ::std::option::Option<::std::string::String>,
    ::aws_smithy_http_server::protocol::rest_json_1::rejection::RequestRejection,
> {
    (!body.is_empty())
        .then(|| {
            let body_str = std::str::from_utf8(body)?;
            Ok(body_str.to_string())
        })
        .transpose()
}

What RequestRejection do we yield? We're leveraging this #[from] conversion when using ? above:

smithy-rs/rust-runtime/aws-smithy-http-server/src/protocol/rest_json_1/rejection.rs

Lines 150 to 151 in dcf16ac

	#[error("request URI cannot be percent decoded into valid UTF-8")]
	PercentEncodedUriNotValidUtf8(#[from] core::str::Utf8Error),

Thus returning an incorrect rejection. We're not percent-decoding a URI here!