Notify collators about seconded collation (#2430)

* Notify collators about seconded collation

This pr adds functionality to inform a collator that its collation was
seconded by a parachain validator. Before this signed statement was only
gossiped over the validation substream. Now, we explicitly send the
seconded statement to the collator after it was validated successfully.

Besides that it changes the `CollatorFn` to return an optional result
sender that is informed when the build collation was seconded by a
parachain validator.

* Add test

* Make sure we only send `Seconded` statements

* Make sure we only receive valid statements

* Review feedback