[example] Create a payjoin example

[ValuedMammal]

Having a payjoin example here will be nice.

Refer to: bitcoindevkit/bdk#913 (comment)
Refer to: bitcoindevkit/bdk-cli#156

Originally posted by @evanlinjin in #1 (comment)

[DanGould]

There's somewhat of an example here: https://github.com/LtbLightning/payjoin-ffi/blob/fa47ff7b798ba5a19900751e277ade71208d2f77/tests/bdk_integration_test.rs

Though I'm not sure how long it'll stay up to date. We'll implement it downstream in python and remove the rust soon enough.

[aagbotemi]

Kindly assign this issue to me.

[DanGould]

We've got a 1.0 release candidate out now that you probably want to build this on. Check out the reference implementation for the latest example, or the integration tests. Feel free to drop me an email or join the payjoin.org discord for any help.