Add "search" optional param and docs for V4

This adds an optional filter param for search on Issues V4 API while also updating the docs.

Notice that search is supported since V3 API (as we filter issues using IssuesFinder), but we had no Grape optional param or docs or tests for it.

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing

What are the relevant issue numbers?

Closes #30195 (closed)