fix Request::into_http, use in handle

[rvolosatovs]

Refs #11843 (comment)

• Verify content-length of bodies originating from the host (even though Request::new assumes that it's already been validated, guest could construct a request using a host body it received from e.g. handle call)
• Return ErrorCode, rather than anyhow::Error on conversion
• Return the request options as part of conversion
• Move BodyExt trait out of host implementation into body module