Prevent large snark works sneaking in on coinbases

[bkase]

One idea: Block producers who are about to win a slot should snark work the coinbase-relevant transaction
^^ Note: This doesn't work because the block producer won't have enough time to know which snark work is coinbase-relevant until the moment they need to make their block (in the majority of cases)

In the short term (this issue), we'll add a new flag --minimum-block-reward <AMOUNT> to block producers that will not add any coinbases or transactions to blocks if the reward is less than the minimum. Longer term, we'll deprecate this as we add more programmability to snark work and transaction selection strategies

In the long term we'll do #8585 .

This will prevent -- once and for all -- any games with snark workers slipping in large fees for the coinbase if for some reason a block producer doesn't hear all the gossip.