spec: Discuss how to harden the Part dissemination protocol

[cason]

This consideration came after discussing the topic with @jtremback and @greg-szabo.

We need some anti-spam mechanism to harden the Parts dissemination protocol. Essentially, we can be overloaded by invalid (Part) messages sent by anyone that is able to join/subscribe to the propagation channel or topic.

[cason]

At the moment, we only check if the signature of a received Part message is valid and has been produced by the proposer of the associated height and round.

[cason]

There is a section on Spam on gossipsub docs/specs: https://github.com/libp2p/specs/blob/master/pubsub/gossipsub/gossipsub-v1.1.md#spam-protection-measures