Update branch rules type to include squash options for all branches rule

From &15526 (comment 2237136180)

For All Branches which aligns with the project setting, the configuration will remain in GitLab Free. This makes sense as squash settings are already in free and the representation in All Branches is purely a UI change.

• For GitLab Free
  • Return the squash options for the project, when the branch rule is for All Branches.
  • Return nil for other types of branch rules