[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.