Preload max access level when loading group labels

What does this MR do and why?

This MR improves the group-level label search query by preloading the group max access levels for the current user. This prevents firing N+1 queries.

The optimization is behind a feature flag: preload_max_access_levels_for_labels_finder

I tested this with PG.ai, using the gitlab-org group:

RequestStore.begin!

current_user = User.find_by_username("ahegyi")

count = 0
ActiveSupport::Notifications.subscribe 'sql.active_record' do |*|
  count += 1
end

puts LabelsFinder.new(
  current_user,
  group_id: 9970,
  only_group_labels: false,
  include_ancestor_groups: true,
  include_descendant_groups: true
).execute.limit(100).pluck(:id)

puts "Query count: #{count}"

Query count (with and without the feature flag): 4 vs 237

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