Allow to filter work items by author on GraphQL

What does this MR do and why?

Allow filtering work items by author username on Graphql.

The field authorUsername is marked as alpha for now until we are 100% sure that we are not going to turn AUTHOR into a widget, more information
at #339777 (comment 1252683756).

Related to #382265

How to set up and validate locally

Create an issue using the UI and execute the GraphQL query from below:

query filteredWorkItems {
  project(fullPath: "PROJECT_FULL_PATH") {
    workItems(authorUsername: "ISSUE_AUTHOR_USERNAME") {
      nodes {
        title
        webUrl
      }
    }
  }
}

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