For the initial WebSocket handshake, we need to create a proper
handshake request and `fetch()` it. Creating this handshake request
begins in `WebSocketMethods::Constructor()`, which sends the partially
prepared `RequestBuilder` over to `components/net/resource_thread.rs`.
There we handle the incoming `CoreResourceMsg::Fetch` message and finish
creating the handshake request in
`components/net/websocket_loader.rs::create_handshake_request()`.
Finally we fetch the request by calling
`components/net/fetch/methods.rs::fetch()`.

`fetch()` eventually calls `http_network_fetch()`. This is where the
"actual fetching" of the request takes place on a network level, which
means we need to handle WebSocket handshake requests differently than
non-WebSocket requests. I mostly moved the existing code, which uses
`tungstenite`, with some type massaging (thanks again, @jdm, for helping
me out here!). This included converting from tungstenite types to
Servo's net types (request/response).

# The tricky bits

In order to fetch the handshake request via
`components/net/fetch/methods.rs::fetch()`, we need to convert the request
URL's scheme from ws(s) to http(s). Then, we need to "undo" this
conversion again when doing CSP checks (i.e. http(s) back to ws(s)). To
avoid having this "undoing" logic in a bunch of places we introduced
`Request::original_url()`, holding the URL before the scheme conversion
took place. Unfortunately this only gets as so far. There are still some
places, where we need to explicitly check and/or convert the URL scheme
(e.g. retroactively upgrading to a secure scheme).

# Related

* https://websockets.spec.whatwg.org/#concept-websocket-establish
* https://fetch.spec.whatwg.org/#http-network-fetch
* w3c/webappsec-csp#532

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Fixes: servo#35028

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Signed-off-by: pylbrecht <pylbrecht@mailbox.org>