Add Cmix struct

Sources/ElixxirDAppsSDK/Cmix.swift

+import Bindings
+
+public struct Cmix {
+  // TODO:
+}
+
+extension Cmix {
+  public static func live(_ bindingsCmix: BindingsCmix) -> Cmix {
+    Cmix()
+  }
+}
+
+extension Cmix {
+  public static let unimplemented = Cmix()
+}