Introduce basic slot-based collator

[skunert]

Part of #3168
On top of #3568

Changes Overview

• Introduces a new collator variant in cumulus/client/consensus/aura/src/collators/slot_based/mod.rs
• Two tasks are part of that module, one for block building and one for collation building and submission.
• Introduces a new variant of cumulus-test-runtime which has 2s slot duration, used for zombienet testing
• Zombienet tests for the new collator

Note: This collator is considered experimental and should only be used for testing and exploration for now.

Comparison with lookahead collator

• The new variant is slot based, meaning it waits for the next slot of the parachain, then starts authoring
• The search for potential parents remains mostly unchanged from lookahead
• As anchor, we use the current best relay parent
• In general, the new collator tends to be anchored to one relay parent earlier. lookahead generally waits for a new relay block to arrive before it attempts to build a block. This means the actual timing of parachain blocks depends on when the relay block has been authored and imported. With the slot-triggered approach we are authoring directly on the slot boundary, were a new relay chain block has probably not yet arrived.

Limitations

• Overall, the current implementation focuses on the "happy path"
• We assume that we want to collate close to the tip of the relay chain. It would be useful however to have some kind of configurable drift, so that we could lag behind a bit. slot-based-collator: Allow slot drift #3965
• The collation task is pretty dumb currently. It checks if we have cores scheduled and if yes, submits all the messages we have received from the block builder until we have something submitted for every core. Ideally we should do some extra checks, i.e. we do not need to submit if the built block is already too old (build on a out of range relay parent) or was authored with a relay parent that is not an ancestor of the relay block we are submitting at. slot-based-collator: Collation & block-builder communication #3966
• There is no throttling, we assume that we can submit velocity blocks every relay chain block. There should be communication between the collator task and block-builder task.
• The parent search and ConsensusHook are not yet properly adjusted. The parent search makes assumptions about the pending candidate which no longer hold. slot-based-collator: Adjust ConsensusHook and parent search #3967
• Custom triggers for block building not implemented.

TBD

• PRDOC

[sandreim]

You also need to enable elastic-scaling-experimental feature when building polkadot-parachain-bin and you should be able to collate properly on all assigned cores.

[skunert]

We could consider iterating from the leaves along the parents here. (not in this PR)

[paritytech-cicd-pr]

The CI pipeline was cancelled due to failure one of the required jobs.
Job name: cargo-clippy
Logs: https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/5844325

[skunert]

Closing in favor of #4097, which is based on this repo and nor my fork. Makes follow up PRs and collaboration easier.