add onboard fee getter script

cadence/scripts/bridge/get_onboard_fee.cdc

@@ -0,0 +1,10 @@
+import "FlowEVMBridgeConfig"
+
+/// Returns the current onboard fee for onboarding an asset to the bridge
+///
+/// @return The onboard fee to be paid in FlowToken
+///
+access(all)
+fun main(): UFix64 {
+    return FlowEVMBridgeConfig.onboardFee
+}