Aura: Broadcast empty step messages instead of creating empty blocks (#7605)

* aura: broadcast empty step message instead of sealing empty block

* aura: add empty_step messages to seal

* aura: include parent_hash in empty step message

* aura: verify received empty step messages

* aura: verify empty step messages in block

* aura: fix dead lock on empty_steps

* aura: fix EmptyStep Encodable

* aura: take number of empty steps into account in chain score

* aura: use empty step signers for finality

* aura: add empty "empty step" messages to seal when reading from spec

* aura: fix EmptyStep rlp encoding

* aura: use Vec<u8> instead of Bytes

* aura: fix block empty step verification

* Update .gitlab-ci.yml

fix lint

* aura: fix accumulation of empty step signatures for finality

* aura: include empty steps in seal signature

* aura: configurable max number of empty steps

* engine: pass block header to seal_fields method

This is necessary to make the number of seal fields dynamic,
e.g. activating a transition on a certain block number that changes
the seal.

* aura: add transition to enable empty step messages

* aura: clear old empty step messages on verify_block_external

* aura: ignore empty step messages from the future

* aura: report skipped primaries when empty steps are not enabled

* aura: fix tests

* aura: report misbehavior

* aura: add tests for rolling finality with multiple signatures

* engine: fix validator set test

In this test the block validation wasn't failing because the block was in the
future (expected failure) but was instead failing because the author of the
block isn't the expected authority. Since we added reporting of blocks produced
by the wrong authority this test started failing.

* aura: reward all the authors of empty step messages

* aura: fix reward attribution for new blocks

* aura: add tests for empty steps broadcasting and inclusion in blocks

* aura: reduce size of empty step messages in seal

* aura: add test for empty step inclusion in blocks

* aura: add test for rewarding of empty steps

* aura: add test for empty steps validation

* aura: fix rlp encoding of sealed empty step

* aura: fix grumbles