slot-based-collator: Allow multiple blocks per slot (#7569)

**Summary:** This PR enables authoring of multiple blocks in one AURA
slot in the slot-based collator and stabilizes the slot-based collator.

## CLI Changes
The flag `--experimental-use-slot-based` is now marked as deprecated. I
opted to introduce `--authoring slot-based` instead of just removing the
`experimental` prefix. By introducing the `authoring` variant, we get
some future-proofing in case we want to introduce further options.

## Change Description
With elastic-scaling, we are able to author multiple blocks with a
single relay-chain parent. In the initial iteration, the interval
between two blocks was determined by the `slot_duration` of the
parachain. This PR introduces a more flexible model, where we try to
author multiple blocks in a single slot if the runtime allows it.

The block authoring loop is largely the same. The
[`SlotTimer`](https://github.com/paritytech/polkadot-sdk/blob/f1935bd9/cumulus/client/consensus/aura/src/collators/slot_based/slot_timer.rs#L48-L48)
now lives in a separate module and is updated with the last seen [core
count](https://github.com/paritytech/polkadot-sdk/blob/f1935bd9

/cumulus/client/consensus/aura/src/collators/slot_based/block_builder_task.rs#L231-L231).
It will then trigger rounds in the block-building loop based on the core
count.

This allows some flexibility where elastic-scaling chains can run on a
single core in quiet times. Previously, running on 1 core with a 3-core
elastic-scaling chain would result in authors getting skipped because
the `slot_duration` was too low.

## Parameter Considerations
The core logic does not change, so there are a few things to consider:
- The `ConsensusHook` implementation still determines how many blocks
are allowed per relay-chain block. So if you add arbitrary cores to an
async-backing, 6-second parachain, `can_build_upon` in the runtime will
deny block-building of additional blocks.
- The `MINIMUM_PERIOD` in the runtime needs to be configured to allow
enough blocks in the slot. A "classic" configuration of
`SLOT_DURATION/2` will lead to slot mismatches when running with 3
cores.
- We fetch available cores at least once every relay chain block. So if
a parachain runs with a 12-second slot duration and 1 fixed core, we
would still author 2 blocks if the parachain runtime allows it.

---------

Co-authored-by: cmd[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Michal Kucharczyk <1728078+michalkucharczyk@users.noreply.github.com>
Co-authored-by: Javier Viola <javier@parity.io>
Co-authored-by: Bastian Köcher <git@kchr.de>