expose estimate_fee api

bdk::blockchain::Blockchain contains an `estimate_fee` api. This commit adds the bindings for estimate_fee.

This will fix https://github.com/bitcoindevkit/bdk-ffi/issues/287

Tested this by adding a unit test in lib.rs. Also generated the python code and verified that it is able to invoke estimate_fee and get the fee rate

bdk-ffi/src/bdk.udl

@@ -164,6 +164,9 @@ interface Blockchain {
   [Throws=BdkError]
   void broadcast([ByRef] PartiallySignedTransaction psbt);
 
+  [Throws=BdkError]
+  FeeRate estimate_fee(u64 target);
+
   [Throws=BdkError]
   u32 get_height();
 

bdk-ffi/src/lib.rs

@@ -305,6 +305,12 @@ impl Blockchain {
         self.get_blockchain().broadcast(&tx)
     }
 
+    fn estimate_fee(&self, target: u64) -> Result<Arc<FeeRate>, BdkError> {
+        let result: Result<FeeRate, bdk::Error> =
+            self.get_blockchain().estimate_fee(target as usize);
+        result.map(Arc::new)
+    }
+
     fn get_height(&self) -> Result<u32, BdkError> {
         self.get_blockchain().get_height()
     }