Add to/from json, and/or serde to Psbt

[riverKanies]

Describe the enhancement

payjoin requires sending a psbt as a string between sender and receiver to build up the payjoin tx.
Currently bdk-wasm does not have a way to extract the psbt as a string or as json, but those functions do exist in bdk-ffi

Use case

payjoin
chain inspection

Additional context

I'm currently building wasm bindings for payjoin and need this functionality. I will add it to my local bdk-wasm and make a PR for this soon

serde is base64, that's what payjoin needs (that's what bitcoin core uses), but it wouldn't hurt to also add json
bitcoin crate exposes Psbt.to_string which is base64, and I believe bdkPsbt should aswell