Added a facility to make DF_REST_Controller->response emit a cache control header.

What I did

• Added a facility to make DF_REST_Controller->response emit a cache control header.
• Added documentation for the new facility.

Implications

• Authors of API endpoints that are subclasses of DF_REST_Controller can specify a default value for cache expiry of all responses (including error responses).
• The value can also be set within individual API methods in that class if required.

Setup

None

How to test

• Code review.
• Check documentation for accuracy and clarity.
• Test with branch feature/prevent_api_response_caching in the main Art Money project.
  • In the admin section, go to your user account screen.
  • Delete all client notes and reload, if you already had any.
  • Check the 404 response when client notes are loaded, and confirm that it includes a cache-control: max-age=0 header.
  • Remove the value declaration from api/v1/client_notes.php and retest, confirming that the 404 response now includes no cache-control header.