Add demo application in kotlin

bindings/bdk-kotlin/demo/src/main/kotlin/Main.kt

@@ -0,0 +1,25 @@
+import uniffi.bdk.*
+
+class LogProgress: BdkProgress {
+    override fun update(progress: Float, message: String? ) {
+        println(progress);
+        println(message);
+    }
+}
+
+fun main(args: Array<String>) {
+    val descriptor =
+        "wpkh([c258d2e4/84h/1h/0h]tpubDDYkZojQFQjht8Tm4jsS3iuEmKjTiEGjG6KnuFNKKJb5A6ZUCUZKdvLdSDWofKi4ToRCwb9poe1XdqfUnP4jaJjCB2Zwv11ZLgSbnZSNecE/0/*)";
+    val db = DatabaseConfig.Memory("")
+    val client = BlockchainConfig.Electrum(ElectrumConfig("ssl://electrum.blockstream.info:60002", null, 5u, null, 100u))
+    val wallet = OnlineWallet(descriptor, Network.TESTNET, db, client)
+    val address = wallet.getNewAddress()
+    println("Please send satoshis to wallet address: $address")
+    readLine()
+    println("Syncing...")
+    wallet.sync(LogProgress(), null)
+    val balance = wallet.getBalance()
+    println("New wallet balance: $balance")
+    println("Press any key to exit")
+    readLine()
+}