Add PVF execution priority (#4837)

Resolves https://github.com/paritytech/polkadot-sdk/issues/4632

The new logic optimizes the distribution of execution jobs for disputes,
approvals, and backings. Testing shows improved finality lag and
candidate checking times, especially under heavy network load.

### Approach

This update adds prioritization to the PVF execution queue. The logic
partially implements the suggestions from
https://github.com/paritytech/polkadot-sdk/issues/4632#issuecomment-2209188695.

We use thresholds to determine how much a current priority can "steal"
from lower ones:
-  Disputes: 70%
-  Approvals: 80%
-  Backing System Parachains: 100%
-  Backing: 100%

A threshold indicates the portion of the current priority that can be
allocated from lower priorities.

For example:
-  Disputes take 70%, leaving 30% for approvals and all backings.
- 80% of the remaining goes to approvals, which is 30% * 80% = 24% of
the original 100%.
- If we used parts of the original 100%, approvals couldn't take more
than 24%, even if there are no disputes.

Assuming a maximum of 12 executions per block, with a 6-second window, 2
CPU cores, and a 2-second run time, we get these distributions:

-  With disputes: 8 disputes, 3 approvals, 1 backing
-  Without disputes: 9 approvals, 3 backings

It's worth noting that when there are no disputes, if there's only one
backing job, we continue processing approvals regardless of their
fulfillment status.

### Versi Testing 40/20

Testing showed a slight difference in finality lag and candidate
checking time between this pull request and its base on the master
branch. The more loaded the network, the greater the observed
difference.

Testing Parameters:
-  40 validators (4 malicious)
-  20 gluttons with 2 seconds of PVF execution time
-  6 VRF modulo samples
-  12 required approvals

![Pasted Graphic
3](https://github.com/user-attachments/assets/8b6163a4-a1c9-44c2-bdba-ce1ef4b1eba7)
![Pasted Graphic
4](https://github.com/user-attachments/assets/9f016647-7727-42e8-afe9-04f303e6c862)

### Versi Testing 80/40

For this test, we compared the master branch with the branch from
https://github.com/paritytech/polkadot-sdk/pull/5616. The second branch
is based on the current one but removes backing jobs that have exceeded
their time limits. We excluded malicious nodes to reduce noise from
disputing and banning validators. The results show that, under the same
load, nodes experience less finality lag and reduced recovery and check
time. Even parachains are functioning with a shorter block time,
although it remains over 6 seconds.

Testing Parameters:
-  80 validators (0 malicious)
-  40 gluttons with 2 seconds of PVF execution time
-  6 VRF modulo samples
-  30 required approvals


![image](https://github.com/user-attachments/assets/42bcc845-9115-4ae3-9910-286b77a60bbf)

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Co-authored-by: Andrei Sandu <[email protected]>