Move GenesisBlock(ProvenBlock) to GenesisBlock(SignedBlock))

[sergerad]

I think this is fine for now, but why is the genesis block defined as a proven block? It can't have a ZK proof and technically should always be just a "signed block".

Let's create an issue to fix this.

Originally posted by @bobbinth in #1725 (comment)

// Current state
pub struct GenesisBlock(ProvenBlock);