Shouldn't matter much, but this is run on every produced block so free
performance

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. ...

* Extract most aura logic out to standalone module, make use of these

* Update client/consensus/aura/src/standalone.rs

improve docs

Co-authored-by: Bastian Köcher <git@kchr.de>

* add slot_duration_at

* ".git/.scripts/commands/fmt/fmt.sh"

---------

Co-authored-by: Bastian Köcher <git@kchr.de>
Co-authored-by: parity-processbot <>