Read from stdin

[swarna1101]

Transforms the CLI into a true Unix tool, enabling shell scripting. Users can pipe logs, files, or any command output directly into publish.

Details: Detect when --message is not provided and read from os.Stdin. Support both --message and stdin (stdin takes precedence if both provided). This enables: tail -f logs.txt | mump2p publish --topic=logs, cat data.json | mump2p publish --topic=data --grpc, etc.