Support for new network validation protocol(v3)

[alexggh]

The parachain core team has been working on some optimizations part of paritytech/roadmap#26 for increasing the number of validators doing consensus work, some changes had been required to the network validation protocol, so a new network validation protocol(v3) will be introduced starting with the upcoming polkadot-v1.6.0.

The implementation is backwards compatible, but the optimisations can not be be enabled untill all the validator switched to polkadot version supporting the new protocol.

Relevant pull requests:

• approval-voting improvement: include all tranche0 assignments in one certificate  paritytech/polkadot-sdk#1178
• Approve multiple candidates with a single signature paritytech/polkadot-sdk#1191
• Add feature flag to enable v2 assignments paritytech/polkadot-sdk#2444

High level description of the changes.

In a nutshell, checking the validity of all assignments and approvals for validating parachain blocks, takes a lot of time, so this optimisation reduce the amount of messages the nodes have to check and forward by merging all assignments from tranche0 in a single message and by opportunistically approving more than one candidate with a single signature.

Polkadot-sdk changes:

• A new validation network protocol, to be able
  to transmit that more than one candidate was included in the assignment(ApprovalDistributionMessage::Assignments, ApprovalDistributionMessage::Approvals)
• Plumbing in approval-distribution, approval-voting, dispute-coordination, so that the new messages are correctly processed and accepted as valid.
• Support to translate between v3 and v1/v2 network protocols, so that we can still gossip to nodes with only v1/v2.
• Update the approval-voting to produce AssignmentCertKindV2::RelayVRFModuloCompact assignments for tranche0, if the runtime features tells us it is enable.
• Update the approval-voting to coalesce opportunistically more than one approval vote, up to the maximum specified in the runtime configuration max_approval_coalesce_count

Let me know if you got questions or any more details are needed.

[sandreim]

Heads up, we have initiated the governance process to enable the changes described here on Kusama: https://kusama.polkassembly.io/referenda/392

CC @kamilsa @iceseer