Backing: Candidate Backing

[P1sar]

Issue summary

Candidate Backing will be a subsystem of parachain workflow.
It ensures every parablock considered for relay block inclusion,

• has been seconded by at least one validator, and
• approved by a quorum.

I will produce signed Statements and track statements produced by other validators.
Once sufficient quorum has been agreed that candidate is valid, it will notify Provisioner Subsystem (to be created after finishing this issue). Provisioner will ensure that candidates are sufficiently available before sending them to potential block authors.
Produce as many backable candidates as possible.

• Get validations from candidate validation subsystem and generate appropriate statements.
• All statements will be sent to Statement Distribution (to be created later) subsystem to be gossiped by peers
• When candidate validation decides that a candidate is invalid, and it was recommended to us to second by collator protocol, we penalize the collator who sent that candidate. To penalize the collator, we send a message to collator protocol with candidate hash.

I will improve this issue with more details as other parachain related work progresses and as I understand more specific details using spec, node implementors guide and polkadot codebase.

Sub Issues

• feat(dot/parachain/backing): handle Statement incoming overseer message #3507
• feat(dot/parachain/backing): handle CanSecond incoming overseer message #3505
• feat(dot/parachain/backing): handle Second incoming overseer message #3506
• feat(dot/parachain/backing): handle GetBackedCandidates incoming overseer message #3504
• feat(dot/parachain/backing): handle validated candidate command #3571
• feat(dot/parachain/backing): handle ActiveLeavesUpdate incoming overseer message #3503
• Implement ext_crypto_ecdsa_generate Host API #2951
• Add a runtime function to get async backing parameters #3802
• test(lib/runtime): TestInstance_ParachainHostAsyncBackingParams logs an error #3809
• add a runtime function to get minimum backing votes #3856
• feat(dot/parachain/backing): implement a function to get PoV from validator #3545
• feat(dot/parachain/backing): requests executor parameters from the runtime effective at given relay-parent #3544
• feat(dot/parachain/backing): Implement Statement Table #3471
• feat(dot/parachain/backing): implement Misbehaviour enum using interface  #3601
• feat(dot/parachain/backing): process overseer signals for candidate backing #3644
• Black Box tests for Candidate Backing #3744

Below are sub-issues to update the Candidate backing subsystem to support Async backing + Elastic scaling.

• candidate-backing-subsystem: Modify logic to handle ActiveLeavesUpdate to support elastic scaling #4327
• find core index of candidate parachain using the validity statement #4324
• Change type of group id from ParaID to CoreIndex #4316
• Update GetBackableCandidates struct and logic to process it #4296
• update CanSecond message process logic to support elastic scaling #4329
• candidate-backing: modify logic to process Second message to support elastic scaling #4403
• candidate-backing: modify logic to process Statement message to support elastic scaling #4404
• update logic to process validated candidate command to support elastic scaling #4402

Other information and links

• https://spec.polkadot.network/chapter-anv#sect-candidate-backing
• https://paritytech.github.io/polkadot/book/node/backing/candidate-backing.html