Add validation for restrictions on work item links

Related to #424880 (closed)

What does this MR do and why?

This MR adds the model validation in RelatedWorkItemLink for the default work item link restrictions (added in !133452 (merged)).

Type	Can be related to	Can block	Can be blocked by
Epic	Epic, Issue, Task, Objective, Key Result	Epic, Issue, Task, Objective, Key Result	Epic, Issue, Task
Issue	Epic, Issue, Task, Objective, Key Result	Epic, Issue, Task, Objective, Key Result	Epic, Issue, Task
Task	Epic, Issue, Task, Objective, Key Result	Epic, Issue, Task, Objective, Key Result	Epic, Issue, Task
Objective	Epic, Issue, Task, Objective, Key Result	Objective, Key Result	Epic, Issue, Task, Objective, Key Result
Key Result	Epic, Issue, Task, Objective, Key Result	Objective, Key Result	Epic, Issue, Task, Objective, Key Result

This validation checks that the types being linked are present in the work_item_related_link_restrictions table and returns an error if they are not. Although there will be existing records for this model, the feature is still behind the FF linked_work_items and not available for GA.

How to set up and validate locally

1. Using rails console enable the feature flags

Feature.enable(:linked_work_items)
Feature.enable(:okrs_mvc)

2. Still in rails console create work items of different types

Click to expand

author, project = User.first, Project.first
task_type = WorkItems::Type.find_by(base_type: 'task')
requirement_type = WorkItems::Type.find_by(base_type: 'requirement')
objective_type = WorkItems::Type.find_by(base_type: 'objective')
key_result_type = WorkItems::Type.find_by(base_type: 'key_result')

task = WorkItem.create!(title: "Task", project: project, author: author, work_item_type: task_type)
task = WorkItem.create!(title: "Requirement", project: project, author: author, work_item_type: requirement_type)
objective = WorkItem.create!(title: "Objective", project: project, author: author, work_item_type: objective_type)
key_result = WorkItem.create!(title: "Objective", project: project, author: author, work_item_type: key_result_type)

3. Visit the task http://GDK_URL/<group_path>/<project_path>/-/work_items/<task_iid> and verify that validation works as expected:

Screen_Recording_2023-10-12_at_15.37.15

MR acceptance checklist

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