Allow the python xApp writer to specify a function/handler to be called when the xApp configuration is updated. The configuration is provided to the xApp as a JSON object in a file with a name like config-file.json. The file path is the value of the XAPP_DESCRIPTOR_PATH environment variable. The file contains the "controls" section of the xApp descriptor. In the current version (Cherry) the configuration is stored in the xApp config map.

The framework can detect the change using the Linux kernel feature inotify.

Note: When the xApp registers the handler, the framework should read the initial configuration and call the handler with this initial configuration. After that, the handler would be executed only when the configmap is updated (inotify).

Note: Given that there is some uncertainty if the xApps require something outside the "controls" section, let's return the whole xApp descriptor to the handler for now. We may later limit this to the controls section.