v4 API: Encode state parameter

[n7studios]

Summary

Base64URL encodes the state parameter with the necessary return_to and client_id data required for the ConvertKit hosted universal redirect URL.

Testing

• testGetOAuthURLWithState: Updated test to assert state parameter contains the correct 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)

[noelherrick]

Just a note, the redirect endpoint only accepts the WordPress app (client_id) right now, so if there are others, we should add them to this list so the redirect works.