Remove url crate dep from payjoin

[benalleng]

Which crate is this feature request for?

payjoin

Please describe the feature you'd like to see added.

Following #513 we have removed url::Url from the public api but for an improvement in safety and reduction in bloat from the crate we should remove the url crate entirely and replace it with an internal payjoin::Url that does some minimal string parsing to acheive the same goals as how we use it in our crate now.

The following comment points out some things we will need in this new payjoin::Url. https://github.com/payjoin/rust-payjoin/issues/513#issuecomment-3330709943

[benalleng]

Following #1126 We still have url/idna through our bhttp dep which is enabling its http feature through dep:http in payjoin/_core