Unable to add Custom Matter ZCL template

[LiwaaK]

[tecimovic]

At this point, custom ZCL clusters are stored in the XML files. zcl.json file is a top-level SDK file that is read in to read multiple XML files.

[tecimovic]

We should add a nice description for the users...

[tecimovic]

In all reality, why not allowing zcl.json to load the custom XML? This way we can simply load multiple XML files through one file.

[tecimovic]

This is essentially an error of giving users nice warnings and error messages.