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

bdkFFI.xcframework/ios-arm64/bdkFFI.framework/Headers/bdkFFI-umbrella.h

@@ -0,0 +1,4 @@
+// This is the "umbrella header" for our combined Rust code library.
+// It needs to import all of the individual headers.
+
+#import "bdkFFI.h"

bdkFFI.xcframework/ios-arm64/bdkFFI.framework/Modules/module.modulemap

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