Streamline reading of json and yaml files

[domna]

Many of our readers read json and yaml files to add additional metadata.
Some of these features are made generally available in the pynxtools.utils but reader implementations differ.

1. I think we should streamline yaml and json reading and offer this as a general solution for adding additional keys to an appdef. There is a generic YamlJsonReader already going in that direction, but it is just used by few readers.
2. I think this would also be useful for plugin developers as they can concentrate on writing the actual data conversion and just can use the general handling of additional metadata through pynxtools.