Add GraphQL type to allow retrieval of compliance violations within a group

Why are we doing this work

To implement the next iteration of the "Compliance Dashboard" we should store compliance violations such as a failure of separations of duties.

Thanks to the work done in #347324 (closed) we should now have a collection of compliance violations saved into the new table. This issue should be used to surface these to the GraphQL API.

Proposed Data Structure

query getComplianceViolations($fullPath: ID!, $filters: ViolationFilters, $sort: String) {
  group(fullPath: $fullPath) {
    id
    mergeRequestViolations(filters: $filters, sort: $sort) {
      nodes {
        id
        severity
        reason
        violatingUser {
          id
          name
          username
          state
          avatarUrl
          webUrl
        }
        mergeRequest {
          id
          title
          mergedAt
          milestone
          webUrl
          author {
            id
            name
            username
            state
            avatarUrl
            webUrl
          }
          mergedBy {
            id
            name
            username
            state
            avatarUrl
            webUrl
          }
          committers {
            nodes {
              id
              name
              username
              state
              avatarUrl
              webUrl
            }
          }
          participants {
            nodes {
              id
              name
              username
              state
              avatarUrl
              webUrl
            }
          }
          approvedBy {
            nodes {
              id
              name
              username
              state
              avatarUrl
              webUrl
            }
          }
          ref: reference
          fullRef: reference(full: true)
          sourceBranch
          sourceBranchExists
          targetBranch
          targetBranchExists
          headPipeline {
            detailedStatus {
              id
              icon
              favicon
              text
              label
              group
              tooltip
              hasDetails
              detailsPath
            }
          }
        }
        project {
          id
          avatarUrl
          name
          webUrl
          complianceFrameworks {
            nodes {
              id
              name
              description
              color
            }
          }
        }
      }
    }
  }
}

The ViolationFilters should be a new type like EpicFilters and it should accept the following arguments:

$projectIds: [ID!], $createdBefore: Time, $createdAfter: Time

Sort should use this format and have ASC and DESC values for Severity, Violation (type / user?), Merge request (alphabetical on name), and Date merged.