src: make url host a proper C++ class

- Gives `URLHost` a proper destructor that clears memory
  depending on the type of the host (This fixes a memory leak)
- Hide the host type enums and class layout as implementation details
- Make the `Parse` methods members of `URLHost`
- Turn `WriteHost` into a `ToString()` method on the `URLHost` class
- Verify that at the beginning of a parse attempt, the type is set
  to “failed”
- Remove a lot of `goto`s from the source code 🐢🚀

Backport-PR-URL: #18324
PR-URL: #17470
Fixes: #17448
Reviewed-By: Timothy Gu <[email protected]>

Author: addaleax