Update gRPC_Security_Cheat_Sheet by adding example for preventing resource exhaustion

[lidiiafedynchuk]

Update gRPC_Security_Cheat_Sheet by adding example for preventing resource exhaustion

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Pull request overview

Updates the gRPC Security Cheat Sheet to address resource exhaustion risks in streaming RPCs by adding guidance and a Go example that attempts to cap streaming message counts and session duration.

Changes:

• Adds a recommendation to limit streaming session duration and maximum messages per stream.
• Adds a Go code example demonstrating message-count and duration limiting logic for streaming.

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[lidiiafedynchuk]

@szh could you please review when you get a chance. Thank you.