Allow creating transactions with dust outputs

Add TxBuilder::allow_dust() that skips checking the dust limit

src/wallet/mod.rs

@@ -790,7 +790,10 @@ where
         let recipients = params.recipients.iter().map(|(r, v)| (r, *v));
 
         for (index, (script_pubkey, value)) in recipients.enumerate() {
-            if value.is_dust(script_pubkey) && !script_pubkey.is_provably_unspendable() {
+            if !params.allow_dust
+                && value.is_dust(script_pubkey)
+                && !script_pubkey.is_provably_unspendable()
+            {
                 return Err(Error::OutputBelowDustLimit(index));
             }
 
@@ -5408,6 +5411,30 @@ pub(crate) mod test {
         builder.finish().unwrap();
     }
 
+    #[test]
+    fn test_allow_dust_limit() {
+        let (wallet, _, _) = get_funded_wallet(get_test_single_sig_cltv());
+
+        let addr = wallet.get_address(New).unwrap();
+
+        let mut builder = wallet.build_tx();
+
+        builder.add_recipient(addr.script_pubkey(), 0);
+
+        assert!(matches!(
+            builder.finish().unwrap_err(),
+            Error::OutputBelowDustLimit(0)
+        ));
+
+        let mut builder = wallet.build_tx();
+
+        builder
+            .allow_dust(true)
+            .add_recipient(addr.script_pubkey(), 0);
+
+        assert!(builder.finish().is_ok());
+    }
+
     #[test]
     fn test_fee_rate_sign_no_grinding_high_r() {
         // Our goal is to obtain a transaction with a signature with high-R (71 bytes