Artifacts: Show total artifacts size for a project

Problem to solve

We removed the artifacts finder call, which provided the total artifacts size, in !102797 (merged), but we should still display this size on the artifacts page.

Proposal

From !102797 (comment 1158166214):

It looks like the usage quota page uses this graphql request, and it is pretty fast:

{
  project(fullPath:"gitlab-org/gitlab") {
    statistics {
      buildArtifactsSize
    }
  }
}