Memoize CI config node validator to prevent leaks

What does this MR do?

This MR memoizes CI configuration node validator, to prevent possible memory leak described in #20698 (closed).

Why was this MR needed?

See #20698 (closed)

What are the relevant issue numbers?

Closes #20698 (closed)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together