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/cli...