Add gossipsub with a default config

[matjazv]

Description

Introduce gossipsub module into libp2p module by implementing following requirements:

• Peer is able to broadcast information/message to all connected peers
• Implement/update unittests.

Implement and use features which are useful for new P2P package. Features to be considered (list is not complete):

• Outbound mesh quotas
• Explicit peering agreements
• List of bootstrapper nodes
• Opportunistic grafting
• Prune peer eXchange
• Extended message validators
• Peer scoring
• Peer banning

Acceptance Criteria

• P2P module supports above features from gossipsub module.
• Peers are able to communicate using gossipsub features (explicit peering agreements, peer scoring/banning, outbound mesh quotas, etc.).
• Peer can broadcast message to all connected peers in the same mesh.
• All new code is unit tested.

Additional Information

• https://github.com/libp2p/specs/tree/master/pubsub/gossipsub