Map<Object, Object> should be persistable

[JanSchankin]

We are unable to persist Maps which have non primitve types (i.e. their wrapper types) as key. We need a new BeanConverterExtenstion that is able to convert and revert Maps of the type Map<Object, Object> in a way that the Objects are converted to PersistentBeans.

We are currently able to persist Map<T, Object> where T is any wrapper of a primitve type. This is done by MapPrimitiveKeyExtension. This can be used as a template.
https://github.com/ppi-ag/deep-sampler/blob/645e0ab8f4775d194d6c90a54a94cd3e72cf8ca9/deepsampler-persistence/src/main/java/de/ppi/deepsampler/persistence/bean/ext/MapPrimitiveKeyExtension.java

MapPrimitiveKeyExtension converts the key to a String, so that the key can be used as a property name in JSON:

{
   "key": "value"
}

There is no nice way to convert complex objects to Strings that could be used as a key in the same way, so we need to find another way to express the key in a persistable way. One way to do it, would be to convert the Map in a List which has a tuple as entry. The tuple would then contain the key and the value.