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

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

@@ -250,6 +250,9 @@ class Blockchain(
     /** Broadcast a transaction. */
     fun broadcast(psbt: PartiallySignedBitcoinTransaction) {}
 
+    /** Estimate the fee rate required to confirm a transaction in a given target of blocks. */
+    fun estimateFee(target: ULong): FeeRate {}
+
     /** Get the current height of the blockchain. */
     fun getHeight(): UInt {}