Add listing of top level groups to BulkImportsController

What does this MR do?

This MR:

• Adds Gitlab::BulkImport::Client to fetch top level groups from source Gitlab instance in order to import them (this will be done in the future MRs)
• Implements only backend. Frontend is done in #249157 (closed)

Rails console example output:

client = Gitlab::BulkImport::Client.new(uri: "http://127.0.0.1:3000", token: "Yoh-")

serializer = BaseSerializer.new(current_user: User.first)

serializer.represent(client.get('groups', page: 1), {}, Import::BulkImportEntity)

[23] pry(main)> serializer.represent(client.get('groups', page: 1), {}, Import::BulkImportEntity)
  ApplicationSetting Load (0.6ms)  SELECT "application_settings".* FROM "application_settings" ORDER BY "application_settings"."id" DESC LIMIT $1  [["LIMIT", 1]]
=> [{:id=>434, :full_name=>"092s4", :full_path=>"092s4"},
 {:id=>342, :full_name=>"0atco", :full_path=>"0atco"},
 {:id=>284, :full_name=>"0ee3x", :full_path=>"0ee3x"},
 {:id=>505, :full_name=>"0eflo", :full_path=>"0eflo"},
 {:id=>152, :full_name=>"0ji7z", :full_path=>"0ji7z"},
 {:id=>489, :full_name=>"0qtpw", :full_path=>"0qtpw"},
 
 ...

Mentions #249156 (closed) & &4374 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content