Allow custom schemes to have non-opaque origins

• Note that this crate implements the URL Standard not RFC 1738 or RFC 3986

Describe the bug

All schemes that are not on the standard list get an opaque origin from https://github.com/servo/rust-url/blob/a66f4220895c3cc84ae623c218466710eb3a812f/url/src/origin.rs#L17 . This makes all origin comparison fail, which in turn causes issues such as CSP violations being reported because 'self' never matches.

It would be very useful to also be able to specify a list of schemes that are not standard ones but can also get a tuple origin. If we reuse the Origin::Tuple() for these we also need to let them provide a default port (see https://github.com/servo/rust-url/blob/a66f4220895c3cc84ae623c218466710eb3a812f/url/src/parser.rs#L186).

I guess that ideally custom schemes should bring their own origin type and we should refactor the whole Origin type to be trait based.

Start with url/src/origin.rs and url/src/parser.rs, especially the linked origin selection and default-port logic. Define how callers specify custom schemes and their default ports, and determine whether the existing Origin::Tuple variant is sufficient or a trait-based Origin redesign is required. Done means custom schemes can receive non-opaque origins and origin comparisons work as intended.