Add AllowDynamicProperties attribute to xPDO class

[opengeek]

This avoids the deprecation warning when dynamic properties are assigned to the class in 8.2+ and resolves #256

[sonicpunk]

We can confirm this behaviour!

[modxcommunity]

This pull request has been mentioned on MODX Community. There might be relevant details there:

https://community.modx.com/t/migxloopcollection-error-on-php-8-2/8051/2