Update add_subscriber_to_sequence() to support name, custom fields and tags

[n7studios]

Summary

Updates the add_subscriber_to_sequence() API function, to include support for first name, custom fields and tags, per https://developers.convertkit.com/#add-subscriber-to-a-sequence.

Adds tests to improve test coverage of new function arguments.

Testing

• testAddSubscriberToSequenceWithFirstName: Test that adding a subscriber to a sequence with a first name specified works.
• testAddSubscriberToSequenceWithCustomFields: Test that adding a subscriber to a sequence with a custom field works.
• testAddSubscriberToSequenceWithTagID: Test that adding a subscriber to a sequence with a tag ID works.
• testGetSequenceSubscriptionsWithDescSortOrder: Test that fetching a subscriber's sequences in descending order works.
• testGetSequenceSubscriptionsWithInvalidSortOrder: Test that a ClientException is thrown when an fetching a subscriber's sequences with an invalid sort order.

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
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• 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)