Add slim query parameter

[n7studios]

Summary

Adds support for the slim parameter on the get_subscribers and get_form_subscriptions methods, which was added to the API on May 21st 2026:
https://developers.kit.com/changelog#may-21-2026-%E2%80%94-subscribers

Testing

• testGetFormSubscriptionsWithSlimParameter: Test that get_form_subscriptions() returns the expected data when the slim parameter is specified.
• testGetSubscribersWithSlimParameter: Test that get_subscribers() returns the expected data when the slim parameter is specified.

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• The code passes when running PHPStan
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)