v4 API: Use Trait for Methods

[n7studios]

Summary

Creates a new trait called ConvertKit_API_Traits class, containing:

• all non-authentication API methods, moved from ConvertKit_API,
• abstract trait methods for authentication and request methods, which ensures classes implementing this trait adhere to to the same structure (i.e. our ConvertKit_API class).

This allows an API class implementation (whether this SDK's ConvertKit_API or our WordPress Libraries) to use the API methods with their own HTTP request libraries (Guzzle for this SDK; wp_remote_* methods for the WordPress Library).

Testing

Existing tests 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
• 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)