Add top_level_only argument to a group resolver

What does this MR do and why?

1. Add top_level_only argument to a groups resolver
2. Graphql group resolver now has top_level_only argument and it is possible to return only top level groups

References

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Description	UI
New argument

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Go to explorer http://gitlab.localdev:3000/-/graphql-explorer

2. Try to query groups/subgroups with top_level_only argument:

   {
     groups(topLevelOnly: true) {
       nodes {
         id
         name
         fullName
         fullPath
         avatarUrl
         parent {
           name
         }
       }
       pageInfo {
         hasNextPage
       }
     }
   }
   

Related to #504374 (closed)