Expose timelog categories

What does this MR do and why?

This MR is a follow up from !87529 (merged) and aims at exposing the timelog categories on namespaces.

All changes are behind the new :timelog_categories feature flag.

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How to set up and validate locally

1. Enable the feature flag from the Rails console

Feature.enable(:timelog_categories)

2. Add a new timelog category from the Rails console

c = TimeTracking::TimelogCategory.new()
c.namespace_id = 31 # This ID should be the one for the FlightJS group, used on the GraphQL query down below
c.name = "Test"
c.save()

3. Use the following GraphQL query to get the timelog categories from the namespace

query {
  namespace(fullPath: "FlightJS") {
    timelogCategories {
      nodes {
        id
        name
        billable
      }
    }
  }
}

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.