Fix get_subscriber_id() by email performance

[n7studios]

Summary

When calling get_subscriber_id(), includes the email_address as an argument, instead of attempting to fetch every single subscriber and then iterating through the array of subscribers to find a match.

Fixes:

• get_subscriber_id() causes 429 "Too many requests" API error when total pages > 120 #21
• Remove inefficient subscriber paging #29
• fix #21: add email_address parameter to get_subscriber_id() #22

testGetResource fails, due to code using a third party library that's reliant on the (not recommended) file_get_contents(). A separate PR to fix the code will result in this test passing.

Testing

• testGetSubscriberID: Tests that fetching a subscriber ID by email address works when a valid subscriber email address is supplied
• testGetSubscriberIDWithInvalidEmailAddress: Tests that an InvalidArgumentException is thrown when an invalid email address is supplied
• testGetSubscriberIDWithNotSubscribedEmailAddress: Tests that fetching a subscriber ID by email address returns false when a valid email address is supplied, but is not a subscriber

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)