Define handling of Decimal values

[leoschwarz]

Some properties return Decimal objects which is an arbitrary-precision representation.
The problem is that this is not serializable to most formats without converting to a string or float.

Currently bfabric_read.py for instance will fail to print a JSON document if there is a single Decimal object in the response.

A simple fix would be to add a clause

def convert_suds_type(item: Any) -> int | float | str:
    if isinstance(item, Text):
        return str(item)
    elif isinstance(item, Decimal):
        return float(item)
    return item

but will need to be considered later whether it is really appropriate. If we retain the Decimal we should define serialization etc. to be aware of this type.