feat: add acfGetFields function

[sebastianthulin]

This pull request introduces a new interface for retrieving fields from a specific field group and implements this interface in two classes, along with corresponding tests. Here are the most important changes:

Interface Addition:

• src/Contracts/AcfGetFields.php: Added a new interface AcfGetFields with a method acfGetFields that retrieves fields from a specific field group.

Implementation in Classes:

• src/Implementations/FakeAcfService.php: Implemented the acfGetFields method in the FakeAcfService class, which registers the function call and returns a predefined value.
• src/Implementations/NativeAcfService.php: Implemented the acfGetFields method in the NativeAcfService class, which directly calls the acf_get_fields function.

Testing:

• src/Implementations/FakeAcfService.test.php: Added a new test method testAcfGetFields to verify that the acfGetFields method in FakeAcfService correctly registers the function call with the provided parent parameter.