Allow setting the page cache expiry from controllers

[lukasbestle]

Describe the PR

• It is now possible to set the page cache expiry timestamp from controllers or templates:

$kirby->response()->expires(1234567890); // timestamp
$kirby->response()->expires('2021-12-31');

• When setting cache values, you can now define an absolute expiry timestamp instead of the number of minutes. The number of minutes is still supported and auto-detected.

Ready?

• Added unit tests for fixed bug/feature
• Added in-code documentation (if needed)
• CI passes (runs automatically when the PR is created or run composer ci locally)
  Running locally requires PHPUnit, PHP-CS-Fixer, Psalm, PHPCPD and PHPMD.

• Checked whether the PR needs documentation, if needed added to the release docs checklist

[afbora]

Looks great like the getkirby/getkirby.com#1304 PR 👍

[distantnative]

Two smaller comments. Otherwise looks good, it's a cool feature that makes caching more flexible.

[bastianallgeier]

I really like this! Makes caching so much more flexible.

[bastianallgeier]

@lukasbestle I was about to change the method name, but for some reason there's a local issue in my tests with timestamps.

[lukasbestle]

I've changed the method name and also added the feature to pass the number of minutes to the expires() method for consistency with the Cache classes.