Support fallback rules

[joao-paulo-parity]

Problem: https://github.com/paritytech/polkadot/blob/3dd9741e77e2ff121990208f6ee1e115adb7e5bf/.github/pr-custom-review.yml#L23 is awkward because it means "all files except ones managed by other rules" and this is achieved by manually excluding conditions from other rules; there should be an automatic and simpler way of achieving the same.

Solution: Allow for a fallback-rule

rules:
  - name: Foo files
    check_type: changed_files
    condition: ^foo/.*
    all_distinct:
      - min_approvals: 1
        teams:
          - foo-team

# The fallback rule matches every file by default and is only attempted if no
# rule is matched for a given file; as it matches every file, it cannot have a
# condition.
fallback-rule:
  min_approvals: 2
  teams:
    - bar-team

Or, more flexibly, allow for a sequence of fallback rules in which the rules are processed in order and stop at the first matched one.

rules:
  - name: Foo files
    check_type: changed_files
    condition: ^foo/.*
    all_distinct:
      - min_approvals: 1
        teams:
          - foo-team

# The fallback rules are processed in order and stop at the first match.
# Fallback rules can't have "exclude" since the processing automatically
# falls through to the next one.
fallback-rules:
  - name: Bar files
    check_type: changed_files
    condition: ^bar/.*
    min_approvals: 2
    teams:
      - bar-team
  # The last rule can't have a condition since it matches everything by
  # default.
  - name: Baz files
    check_type: changed_files
    min_approvals: 2
    teams:
      - baz-team

[mordamax]

We don't have a use-case for that