Replace Configuration with RequestBuilder to replace PjUri::create_pj_request

[DanGould]

          If this configuration is going to hold this data then yes, I think it makes sense to be part of this. The idea of a builder to expose this interface in an accessible way rather than expose `optional_parameters::Params` as pub makes sense to me.

I'm starting to think of this Configuration struct more as a sender::RequestBuilder that can turn into a (Request, Context) when you call from_psbt_and_uri. Maybe the signature for that method should be

impl RequestBuilder {
    fn create_pj_request(psbt: Psbt, pj_url: Uri<PayJoin>) -> Result<(Request, Context)>;
}

instead of on the manually-validated Uri<PayJoinParams> how it is now: pj_url.create_pj_request(psbt, pj_params: Configuration) -> Result<(Request, Context)>.

Then create_pj_request could validate the endpoint check_pj_supported() during that call instead of requiring it be done by hand before the call, too.

Originally posted by @DanGould in #18 (comment)

[DanGould]

Right now this crate re-exports bip21 types. I think these could be hidden by using a builder, accepting only a string and reporting an error for bip21 without payjoin support.

[DanGould]

This work would also simplify the api for v2 in #101. I'd like to return impl send::Context to support both versions' contexts but can't within yet another trait impl for PjUri, but I could if this were an impl that behaved like RequestBuilder::build() -> (Request, impl send::Context)