Simplify build.sh and add template bdkFFI.xcframework to repo

bdkFFI.xcframework/macos-x86_64/bdkFFI.framework/Modules/module.modulemap

@@ -0,0 +1,6 @@
+framework module bdkFFI {
+  umbrella header "bdkFFI-umbrella.h"
+
+  export *
+  module * { export * }
+}