Initialize Response with InMemory external underlying source by default and export another method for FetchResponse

[CYBAI]

While working on #29640, I've noticed that we're initializing Response with ExternalUnderlyingSource::FetchResponse by default. However, in most cases, when Response constructor is used from JS, it should always use InMemory external underlying source instead, even it doesn't have a body. Also, FetchResponse should only be used by the browser engine itself when it's used by fetch.

servo/components/script/dom/response.rs

Lines 56 to 61 in 98bae01

	pub fn new_inherited(global: &GlobalScope) -> Response {
	let stream = ReadableStream::new_with_external_underlying_source(
	global,
	ExternalUnderlyingSource::FetchResponse,
	);
	Response {

Thus, it would be better to initialize the Response::new with InMemory by default and we should export another method to initialize Response with FetchResponse for fetch.

We can start to work on this when #29640 is merged.