Add get_subscriber_stats method

[n7studios]

Summary

Adds a method for fetching a subscriber's stats.

Testing

• testGetSubscriberStats: Test that get_subscriber_stats() returns the expected data when using a valid subscriber ID.
• testGetSubscriberStatsWithInvalidSubscriberID: Test that get_subscriber_stats() 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)