Make on_idle WeightMeter factor in block size

[skunert]

We want to get rid of the current defensive limit on pov size defined here:

polkadot-sdk/cumulus/client/consensus/aura/src/collators/lookahead.rs

Line 394 in cdb297b

(validation_data.max_pov_size / 2) as usize,

With #4326 in place we are already close.
There is one more scenario we need to fix before we can safely enable the full size there. It can happen that on_idle consumes more weight than what is available and exceed the PoV size.

Example for this scenario:

• The check introduced in CheckWeight SE: Check for extrinsic length + proof size combined #4326 reports that that no more extrinsics can be included in a block because weight + block length exceed the PoV size.
• One or more pallets have an on_idle hook defined that gets called with the rest weight. However, since block length and storage proof weight are tracked separately, the remaining weight passed to on_idle will be too high.

Solutions:

1. We unify the tracking of block length and storage proof weight. This needs adjustment of the check introduced in CheckWeight SE: Check for extrinsic length + proof size combined #4326. If we go this route one needs to double check the pov-reclaim logic for adjustments.
2. We subtract the block length in frame-executor before we calculate the remaining weight to be passed in to the hook. This is more of a quick fix, while 1. would tackle the problem globally.