Add constraint for User Role Condition roles so it can be FullyValidatable

Added ConfigExists constraint to user_role condition schema

meghasharma → closed merge request !12279

I’ve assigned this issue to myself. I initially made the changes in the XB core's user.schema.yml, but later realized that the changes should go into Drupal core. I’d appreciate some guidance on how to proceed and push the changes to the correct branch.

I pushed the changes to the Drupal core branch and updated the issue status to 'Needs review'.

Has test failures.