v4 API: Use OAuth instead of API Key and Secret

[n7studios]

Summary

Replaces the API Key and Secret fields with the OAuth flow using the v4 API at Settings > ConvertKit, using the v4 API library implementation for API requests.

Before:

After:

Video:
https://www.loom.com/share/e5f412bbf9954bb6bbd8ad367a4ce20f?sid=2cd70890-aadb-4faf-9d1b-bc0d8ae2e804

Testing

Some tests will currently fail due to changes in the UI for OAuth, Setup Wizard and Legacy Forms / Landing Pages.

Further PR's submitted for ease of review to cover these separately:

• Tests: v4 API: Tests: Use OAuth instead of API Key and Secret #653
• Exchanging API Key and Secret for Access and Refresh Token: v4 API: Exchange API Key and Secret for Access and Refresh Tokens on Plugin Upgrade #656
• Legacy Forms and Landing Pages: v4 API: Legacy Resources #663
• Setup Wizard (to follow, still WIP)

With those PR's merged, tests here must then pass.

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)