Runtime light client consensus verification

[kostko]

Now that we have all the infrastructure available, the runtime (when running outside the node's trust domain, e.g. as an enclave) should verify light client proofs of consensus layer state before proceeding to process runtime transactions. This should include the runtime confirming that its RAK/nonce is indeed registered in the consensus layer.

This requires the runtime to have a hardcoded trust anchor to the consensus layer which is probably a combination of the following:

• Consensus layer chain context.
• Trusted hash of a consensus layer header at a specific height.

One of the questions to explore in the design is how would the runtime handle a breaking consensus layer upgrade where the chain context changes.