API: Add Topics API

What does this MR do and why?

In the UI, there have been some recent enhancements for project topics:

• Thanks to !71033 (merged), an admin can manage topics.
• Thanks to !62443 (merged), a user can explore topics.

Let's reflect these UI enhancements on the API as well: This MR adds a new Topics API with the following endpoints:

• Get topics
• Get topic
• Create topic (admin only)
• Update topic (admin only)

with at Siemens /cc @bufferoverflow

How to set up and validate locally

See documentation included in this MR in doc/api/topics.md with the corresponding sample API requests.

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.

closes #345788 (closed)

spec/requests/api/topics_spec.rb

@@ -7,11 +7,11 @@
   let_it_be(:topic_2) { create(:topic, name: 'GitLab', total_projects_count: 2) }
   let_it_be(:topic_3) { create(:topic, name: 'other-topic', total_projects_count: 3) }
 
-  let_it_be(:file) { fixture_file_upload('spec/fixtures/dk.png') }
-
   let_it_be(:admin) { create(:user, :admin) }
   let_it_be(:user) { create(:user) }
 
+  let(:file) { fixture_file_upload('spec/fixtures/dk.png') }
+
   describe 'GET /topics' do
     it 'returns topics ordered by total_projects_count' do
       get api('/topics')