Add API support for getting description templates

Merged Jan Beckmann requested to merge kingjan1999/gitlab-ee:23664-description-templates-api into master

What does this MR do?

This MR extends the existing project templates API to also support getting issue and merge request templates.

Screenshots

(none)

Does this MR meet the acceptance criteria?

Conformity

Availability and Testing

Closes #23664 (closed)