OAuth2 PKCE and new end point support

Constant Contact is turning off the old endpoints for this library after March 2022. You must update to this version before then. You will also need to create a new application on the Constant Contact Developer Portal.

Due to new authorization requirements, this library now requires sessions to be turned on. You must also change acquireAccessToken pass all $_GET variables in an array rather than just the code variable. See updated readme for all details.

Other improvements include updated Definition classes and stricter type checking.

Thanks to jmalko for testing.