Return a JSON object representing the serialized, base64-encoded PSBT.
psbtstringREQUIREDcurl --user myuser --data-binary '{"jsonrpc":"1.0","id":"1","method":"decodepsbt","params":["psbt_base64"]}' -H 'content-type:text/plain;' http://127.0.0.1:8332/{
"tx": { "txid": "...", "version": 2 },
"global_xpubs": [],
"psbt_version": 0,
"inputs": [...],
"outputs": [...]
}