Added total policy violations count to the GrpahQL API

What does this MR do and why?

Added the total number of policy violations as a type to ghe GraphQL API,

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

How to set up and validate locally

Run a GraphQL query like:

query  {
  project(fullPath: PROJECT_PATH) {
    mergeRequest(iid: IID) {
      policyViolations {
        violationsCount
      }
   }
}