`Receiver::pj_url` is missing fragment parameters

[spacebear21]

According to the docstring:

    // The contents of the `&pj=` query parameter.
    // This identifies a session at the payjoin directory server.
    pub fn pj_url(&self) -> Url {

But the return value only contains the subdirectory (e.g. https://payjo.in/4A8JZNN5SD9EV) and is missing the hpke pubkey, ohttp keys, and expiration in fragment parameters.

[DanGould]

I believe this problem was eliminated in #466 by replacing pj_url with subdir to reflect our use within this workspace. I don't believe we have a reason to get the complete pj_url outside of the pj_uri.

I think we may even be able to get this out of the bip21 URI already by calling uri.extras.endpoint() and uri.extras.is_output_substitution_disabled(), and we could minimally expose this in bindings.

I'm aware we were using this in Bull Bitcoin mobile to display in the bip21 by applying the pj= and pjos=0 explicitly. @spacebear21 do you think we can refactor Bull Bitcoin mobile to use bitcoin_uri, or do we need some way to extract the request URL explicitly?

[spacebear21]

Yeah I think this is obsolete now, we ended up using the pjUriBuilder to display the bip21 anyway. subdir is a more descriptive name that I wouldn't expect to contain fragment parameters.