Provide a tool for doing offline validation

[bkchr]

When working with blockchains it can happen that sometimes there is some non-determinism or similar problems that you don't see when building the block. In traditional blockchains you will see other nodes fail to import the block and it will be "relative" easy to debug this. When it comes to parachains, this can not be that easy. Importing the block maybe works, but it fails while being validated on the relay chain. If you encounter such a problem in a local test network, it is also relative easy to debug as you control all the nodes. However, in a fully decentralized network this gets impossible. You will not have access to the logs of the relay chain validators to check why your candidate failed to validate.

For this use case I think it would be nice to have some sort of offline validation tool. This tool takes a PoV blob, a parachain validation blob, the encoded validation params and tries to execute the PoV.

Later we also should provide a way to get a PoV from a parachain in some "easy" way.