Add Tag API functions

[n7studios]

Summary

Adds the following API functions to provide API coverage for Tags

• get_tags()
• create_tag()
• tag_subscriber()
• remove_tag_from_subscriber()
• remove_tag_from_subscriber_by_email()

Outputs a PHP deprecated notice if using add_tag(), because:

• The function name is misleading; it actually tags a subscriber, rather than creates a tag - using tag_subscriber() is therefore preferred to tag a subscriber,
• The second argument supports an array of options; tag_subscriber() provides the specific supported arguments (tag ID, email address, first name, custom fields)

add_tag() is not removed at this time, to ensure we don't introduce breaking changes to the SDK. The deprecation notice serves as an early warning to developers to migrate to tag_subscriber() as we will remove add_tag() at a later date.

Testing

• Tests for above functions included.

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)