Add "compare_to" to rules:changes

What does this MR do and why?

This is the final step of adding the compare feature to rules:changes. In "compare" you can write the ref that you want to compare with for the changed files.

This MR targets the branch of !90171 (merged)

Step	Status
1. Move CI rules-changes to an entry class	!89463 (merged)
2. Refactor CI rules entry to use composed value	!90238 (merged)
3. Add rules:changes:paths alias to rules:changes	!90171 (merged)
4. Implement rules:changes:compare	👈 You are here

Related to #293645 (closed)

These changes are behind a feature flag ci_rules_changes_compare #366412 (closed).

Screenshots or screen recordings

Example config:

test2:
  script: exit 0
  rules:
    - changes:
        paths:
          - README.md
        compare_to: main

Test 1: Create a new branch with no change

No pipeline is created

Test 2: Create a new branch with a change on another file

No pipeline is created

Test 3: Create a new branch with a change on README.md

Yay 🎉

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.