Add pipeline_artifacts_size to projectSatisticsType

What does this MR do?

It adds pipeline_artifacts_size to GraphQL ProjectStatisticsType, so both project and namespace have the same field when being queried.

The change is needed as part of #334888.

How to setup and validate locally (strongly suggested)

1. Visit /-/graphql-explorer locally
2. Query a project by its path (e.g. group/project) with its statistics

{  
  # add project's fullPath inside the empty quotes ""
  project(fullPath: "") {
    statistics {
      pipelineArtifactsSize
    }
  }
}

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have self-reviewed this MR per code review guidelines.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)