Integrate Plumo proofs into the lightest sync protocol

[lucasege]

Description

WIP

Tentative plan:

• [x] Implement AddProof RPC command for full nodes
• [x] Gossip added proofs to peers
• [x] Add new general les message: "I have val set at epoch X", server responds with correct combination of headers/proofs as available
• [x] Verify proofs within client
• [x] Update light client state based on proof (validator set)
• [ ] Implement lightest client lookbacks - will need the validator set for any epoch we want to query

Other changes

Describe any minor or "drive-by" changes here.

Tested

An explanation of how the changes were tested or an explanation as to why they don't need to be.

Related issues

• Fixes #[issue number here]

Backwards compatibility

Brief explanation of why these changes are/are not backwards compatible.

[hbandura]

@mcortesi should I close this?