Express mempool constraints ito fees

The mempool has a batch and block budget which it uses to constrain the content of the batches and blocks it proposes. Currently, this is expressed in absolute note, transaction and account update amounts but in the future it likely makes more sense to consider them similarly to fee structure.

> Not related to this PR, but I wonder if we should express some of these in terms that are closer to how we think about fees.
>
>For example, instead of the maximum number of transactions, maybe it should be the maximum number of "verification units" (which we currently compute as `log2(num_tx_cycles)`. So, a simple transaction with Falcon signature would be 17 units, and a much more complicated transaction that requires 1M cycles, would be 20 units. And then if we set the budget at 250 verification units, we'd be able to include 15 - 16 regular transactions in a batch, or 11 - 12 "big" transactions etc.
>
> Similar could probably be applied to account updates and input notes - though, this would require more thinking.
>
>cc @PhilippGackstatter.

_Originally posted by @bobbinth in https://github.com/0xMiden/miden-node/pull/1234#discussion_r2378196519_