fix: add shebangs to async SDK implementations

Add #!/usr/bin/env interpreter lines to async SDKs for portability:
- Python: #!/usr/bin/env python3
- JavaScript: #!/usr/bin/env node
- Ruby: #!/usr/bin/env ruby
- PHP: #!/usr/bin/env php

clients/python/async/src/un_async.py

@@ -1,3 +1,4 @@
+#!/usr/bin/env python3
 """
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