Add macOS support

This MR adds support for the macOS platform.

• Update build-bindings script so it builds for iOS, iOS Simulator, and macOS targets.
• Rebuild Bindings.xcframework.

build-bindings.sh

@@ -60,6 +60,7 @@ go get golang.org/x/mobile/bind
 go install golang.org/x/mobile/cmd/gomobile@latest
 gomobile init
 gomobile bind -target $framework_target gitlab.com/elixxir/client/bindings
+
 message "Move framework..."
 cd $working_dir
 rm -rf "$frameworks_dir/Bindings.xcframework"