feat: add gossipsub large message example for 500KB payload delivery timing

[abhi0324]

What was wrong?

There was no focused example showing large-message behavior with GossipSub, making it harder to verify 500KB publish/receive timing in a simple two-peer setup.

How was it fixed?

Added a new example with receiver/sender modes:

• Receiver subscribes and waits for one message
• Sender connects and publishes one 500KB payload
• Receiver prints payload size, sender peer ID, and end-to-end timing

Also:

• Added a CLI entrypoint: gossipsub-large-message-demo
• Updated PubSub docs with run instructions and source include

To-Do

• Clean up commit history
• Add or update documentation related to these changes
• Add entry to the release notes