Add v4 API Key Support

[n7studios]

Summary

Resolves this reported issue, adding support for the named apiKey parameter on the ConvertKit_API class constructor.

Testing

Shared tests have been moved to tests/ConvertKitAPITest.php, with both ConvertKitAPIKeyTest and ConvertKitAPIOAuth tests inheriting them.

These two test classes override some specific tests as required (e.g. bulk endpoints are not supported using an API Key, so tests exist in ConvertKitAPIKeyTest for these to ensure a ClientException is thrown).

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)

[noelherrick]

What value do you see with these tests as they appear to be testing whether or not the Kit API is throwing an exception vs. testing logic in this library? Or am I misunderstanding?

[n7studios]

The bulk API endpoints and purchases endpoints don't support using an API Key, and therefore rightly return an error - so these tests confirm that the PHP SDK throws a ClientException, which I think there's value in doing here.