Adds RelativePositioning concern to WorkItems::ParentLink model

What does this MR do and why?

Adds RelativePositioning to WorkItems::ParentLink. WorkItems::ParentLink model already contains both relative_position and work_item_parent_id attributes.

This is the first part of MR series to enable the ability to manually reorder children in the Work Items Hierarchy Widget:

• No client-facing change to our REST and GraphQL APIs yet. Database queries introduced are not hitting production. Independent GraphQL mutation will be available soon, see #370401 Task Implement GraphQL mutation to update the relative position field for more details.
• Makes positioning methods available for WorkItems::ParentLink. All of them are used nowhere except RSpec tests.
• Extracts a magic number into items_with_nil_position_sample_quantity variable inside a class that supports relative positioning shared examples set. We need to have it configurable because WorkItems::ParentLink validates children quantity and 100 is the current maximum we allow.
• No changelog entry, therefore.

In case this change looks too small from your perspective: feel free to close this MR so I can integrate this everything in my upcoming step towards #370401 Task Implement GraphQL mutation to update the relative position field

How to set up and validate locally

1. Local Test scenario

• Create a new Issue:
  • Add five tasks to the issue named Task #1 (closed), Task #2 (closed) ... Task #5 (closed).
    • Verify new tasks are added to the end of the list, which is consistent behavior.
    • Verify nothing is broken.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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

Related to #370401 (closed)

Related to #370401 Task Add database-related stuff: include RelativePositioning concern and implement two basic methods