Add support for overwritting the configuration from the execution context

I need to be able to change the value of a configuration parameter at runtime, for a limited period of time, while some code is executed. To give an example, I need to disable the server-side image resize during the PDF export, see XWIKI-21763.

 One simple solution is to implement a ConfigurationSource that stores the values on the execution context, and to modify existing configuration components to check this source first. For this to work we also need to add support for removing a configuration property, of course, without forcing all configuration sources to implement it.