Add get_purchase() function

[n7studios]

Summary

Adds a get_purchase() function for the purchases API endpoint.

Testing

• testGetPurchase: Test that fetching a specific purchase works
• testGetPurchaseWithInvalidID: Test that a ClientException is thrown when an invalid purchase ID is specified
• testCreatePurchaseWithMissingData: Test that a ClientException is thrown when creating a purchase with invalid purchase data

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)