Add score grouping to fellows reviews

[Bullrich]

Per proposed in polkadot-fellows/runtimes#93 we need to have a slight change in the review-bot type fellows.

The new solution will keep the old behaviour, with a slight modification:

It will still request a minimum amount of reviews of a given rank, but it will now have a minimum score that it must be achieved also to be approved, allowing more flexible reviews from lower ranks.

For this, a new field named score will be introduced, this will contain a list of all the available dans and their score.

When individuals approve a PR, their dan's score will count towards the total required score.

For example:

score:
  dan1: 1
  dan2: 2
  dan3: 5
  dan4: 9
  dan5: 10
  dan6: 15
rules:
  - name: CI Files
    condition:
      include:
        - ^\.github/.*
    type: fellows
    minRank: 3
    minApprovals: 1
    minScore: 18

This would work in the following way:

CI Files would require at least one approval from someone who is dan 3 or greater. And the combined value of all the approvals, defined in minScore, must get to 18.

So, in this case, a vote of a dan 6, would count 15 points. And we would still need 3 more points. Could either be from:

• 3 dan 1.
• 1 dan2 and 1 dan1.
• 2 dan2.
• 1 dan3 or greater.