Add soloOwned argument to user groups GraphQL field

What does this MR do and why?

Add soloOwned argument to user groups GraphQL field. To be used in Draft: [skip ci] Draft: Show sole-owner groups ... (!257145 - closed)

Query Plan:

• https://console.postgres.ai/gitlab/projects/gitlab-production-main/sessions/58392/commands/163179
• with mock data: https://console.postgres.ai/gitlab/projects/gitlab-production-main/sessions/59322/commands/164499

References

Related to #629289 (closed)

Screenshots or screen recordings

How to set up and validate locally

1. Go to https://gdk.test:3443/-/graphql-explorer and run the query:

{
  currentUser {
    groups(soloOwned: true) {
      count
      nodes {
        id
        name
        fullPath
      }
    }
  }
}```

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