Add examples on how to use the Extras

[benthecarman]

For now can look at @DanGould's

https://github.com/chaincase-app/payjoin/blob/master/payjoin/src/uri.rs

[DanGould]

That's all @Kixunil's work, not mine. Still useful to have examples, especially when this crate gets used in bindings

[DanGould]

I think we could show how library-defined Extras could be composed. For example, bitcoin-walia duplicates payjoin bip21 parsing by defining WaliaExtras as follows:

https://github.com/MutinyWallet/bitcoin-waila/blob/627375a3b628616ccea6d65bd8bc912558fddcf9/waila/src/bip21.rs#L15-L19

and later duplicating the de/serialization logic

Where it could be defined as

pub struct WailaExtras {
    pub lightning: Option<Bolt11Invoice>,
    pub pj_extras: Option<PayjoinExtras>
}

and then use the bip21 defined functions like PayjoinExtras::deserialize_temp within its own
WaliaExtras::deserialize_temp

Right now the payjoin library actually implements 2 separate Extras, PayjoinParams to parse and PayjoinParams to check validity, But I think I'll narrow it to a single PayjoinExtras like this

[Kixunil]

Good point. I think I had some macros in mind to help with this but I don't remember it well.

[DanGould]

Macros would be great to allow builders for Uri<Extras>