Add PartiallySignedTransaction.json_serialize() function

api-docs/kotlin/src/main/kotlin/org/bitcoindevkit/bdk.kt

@@ -369,6 +369,9 @@ class PartiallySignedTransaction(psbtBase64: String) {
      * In accordance with BIP 174 this function is commutative i.e., `A.combine(B) == B.combine(A)`
      */
     fun combine(other: PartiallySignedTransaction): PartiallySignedTransaction
+
+    /** Serialize the PSBT data structure as a String of JSON. */
+    fun jsonSerialize(): String
 }
 
 /**

bdk-ffi/src/bdk.udl

@@ -307,6 +307,8 @@ interface PartiallySignedTransaction {
   u64? fee_amount();
 
   FeeRate? fee_rate();
+
+  string json_serialize();
 };
 
 dictionary TxBuilderResult {

bdk-ffi/src/psbt.rs

@@ -1,6 +1,8 @@
 use bdk::bitcoin::hashes::hex::ToHex;
 use bdk::bitcoin::util::psbt::PartiallySignedTransaction as BdkPartiallySignedTransaction;
+use bdk::bitcoincore_rpc::jsonrpc::serde_json;
 use bdk::psbt::PsbtUtils;
+use std::ops::Deref;
 use std::str::FromStr;
 use std::sync::{Arc, Mutex};
 
@@ -66,6 +68,12 @@ impl PartiallySignedTransaction {
     pub(crate) fn fee_rate(&self) -> Option<Arc<FeeRate>> {
         self.internal.lock().unwrap().fee_rate().map(Arc::new)
     }
+
+    /// Serialize the PSBT data structure as a String of JSON.
+    pub(crate) fn json_serialize(&self) -> String {
+        let psbt = self.internal.lock().unwrap();
+        serde_json::to_string(psbt.deref()).unwrap()
+    }
 }
 
 // The goal of these tests to to ensure `bdk-ffi` intermediate code correctly calls `bdk` APIs.