Use bitreq::Url for LSPS5 webhook URLs

[tnull]

Replace the custom LSPS5 URL parser with bitreq::Url while keeping the LSPS5-specific HTTPS and length checks. This reduces bespoke parsing logic and aligns accepted webhook URLs with the HTTP client's URL handling.

Co-Authored-By: HAL 9000

[ldk-reviews-bot]

👋 Thanks for assigning @TheBlueMatt as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[ldk-claude-review-bot]

I've thoroughly reviewed the entire PR diff, checked wire compatibility, the new Writeable implementations, Hash/Eq consistency, and all behavioral changes. Both issues from my prior review pass have been addressed in the current code.

No new issues found.

No issues found. Both previously flagged concerns have been resolved:

1. Hash/Eq consistency — PartialEq is now manually implemented over as_str(), consistent with Hash.
2. Writeable allocation — self.0.as_str().write(writer) now delegates to the new impl Writeable for &str, which writes length + bytes directly with no heap allocation.

The impl Writeable for &str addition in lightning/src/util/ser.rs is correct — it doesn't conflict with the existing blanket impl<'a, T: Writeable> Writeable for &'a T because str is unsized and doesn't implement Writeable. The String impl now delegates to &str, producing identical wire format.