Feat/mock signing in 2.5

[jferrant]

Partially closes #4984

[kantai]

Just some superficial comments.

Otherwise, yes, I think we should add some more data to the MockSignature struct like:

pub struct MockSignature {
   signature: ...,
   data: MockSignData
}

pub struct MockSignData {
   stacks_tip_consensus_hash: ConsensusHash,
   stacks_tip: BlockHeaderHash,
   server_version: String,
   burn_block_height: u64,
   pox_consensus: ConsensusHash, 
}

Then, yes, its fine if your test just asserts using the burn_block_height.