Add tag_subscribers method

[n7studios]

Summary

Adds a method for bulk tagging subscribers.

Testing

• testTagSubscribers: Test that tag_subscribers() returns the expected data.
• testTagSubscribersWithInvalidTagID : Test that tag_subscribers() throws a ClientException when an invalid tag ID is specified.
• testTagSubscribersWithInvalidSubscriberID : Test that tag_subscribers() throws a ClientException when an invalid subscriber ID 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)

[noelherrick]

This comment makes it seem like tagging with an invalid is possible with OAuth. Can we drop or modify the last clause? Same goes for the comments on the following functions in this file. I'm also wondering if these test are necessary since we're testing the specific behavior in ConvertKitAPITest.php

[n7studios]

They can't be removed, as ConvertKitAPIKeyTest and ConvertKitAPIOAuthTest both extend ConvertKitAPITest, which is where most of the tests are contained (and therefore run) - otherwise, there'd be duplicate tests in two classes.

I've marked these methods to be skipped in ConvertKitAPIKeyTest, given testCreateTags checks for the ClientException.

Other redundant tests skipped in this PR.

[noelherrick]

Good to know on the inheritance constraints - after your explanation, it makes sense that you'd need to implement all parent methods.