mempool, refactor: add `MemPoolBypass`
#25434, #25532 and #25570 add some parameters to bypass some mempool checks. The motivations for each parameter can be found in these PRs.
https://github.com/bitcoin/bitcoin/pull/25434#discussion_r916378609 suggested to allocate the existing mempool parameters bypass_limits and test_accept in the same structure.
This PR makes that change without introducing new behaviors or new assumptions. It is basically a refactor and will be used as basis for the new parameters in the PRs mentioned above.