Sidechain finality missing checks (like InvalidAuthority)

[murerfel]

The sidechain finalization pallet will accept and finalize sidechain blocks that are deemed invalid by a validateer.

In the finalization pallet, we check the signature and ancestry. But we don't check if the block was authored in the correct slot. We had a case of this happen regularly on the sidechain: integritee-network/worker#750
So the pallet accepted and finalized a block that was discarded by the validateers. As a result, finalization stops working, because all the subsequent blocks will not have the correct ancestor, and we eventually get the error

Some extrinsic Failed: Module(PalletError { pallet: "Sidechain", error: "BlockNumberTooHigh", description: ["A proposed block is too early."] })

We might need to somehow share the logic to determine if a sidechain block is valid, because it is important the finalization pallet and the validateers come to the same conclusion about the validity of a sidechain block.

[gaudenzkessler]

We should define, where we keep shared information for pallets and worker such that we do noe need to copy code from one repo to another.

[haerdib]

Check out #50

[gaudenzkessler]

closed in favor of worker#855