Add `Checkpoint` heights

[Nashtare]

This changes the notion of genesis block for a checkpoint block, allowing to pass a None argument not only for the first block post-genesis but at any chosen checkpoint height.

• removed constraint on the checkpoint block number to be 0. It is still checked to be 1 less than the current block proof.
• binds block_proof output PublicValues to what's encoded, similarly to how it's done at the aggregation level, instead of letting them be arbitrary.
• added an example of changing the checkpoint for a new block in log_opcode

@LindaGuiga Would you mind reviewing?

cc @BGluth, this would also allow us to not have to restart from block 1 whenever we want to test block N :)

[sonarcloud[bot]]

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