Add mempool config for minimum block inclusion fee per cost

[Rigidity]

Purpose:

This will allow farmers to decide what the minimum fee per cost ratio they want to include in blocks is, which prevents wasting resources building blocks with fee-less transactions (possibly useful on lower end hardware). Also, as was made clear by the recent fast forward mempool bug, it's important to have mempool diversity.

Current Behavior:

The mempool currently has no configuration options.

New Behavior:

You can now configure the minimum fee per cost for a transaction to be included into a block. However, lower fee transactions will still be received in the local mempool and propagated throughout the network. Fully blocking them from entering the mempool would have more adverse effects at scale.

Testing Notes:

Has not been tested yet, hence being in draft for now.

[github-actions]

File	Coverage	Missing Lines
chia/full_node/mempool.py	88.9%	lines 557

Total	Missing	Coverage
14 lines	Unknown	92%

[arvidn]

we should have tests for this too. Ideally one that uses create_block_generator() as I'm hoping well transition away from first creating the SpendBundle. Perhaps a parameterized one, to ensure a few different ratios work.

In case there isn't an obvious conflict, maybe you can try to rememeber to remind of of this new setting before #19270 lands too

[arvidn]

probably not important; one option is to use the fee_per_cost column from the database, rather than recomputing the ratio. The only reason I can think of to prefer the database field is to be consistent with what actually matters for the purpose of ordering the items. Perhaps this ratio will be computed slightly different in the future.