Allow to define dedicated name for fesa value-items

Currently, the name of the generated Fesa field needs to be identical to the name of the generated value-item, otherwise the code generated by Silecs will be wrong / won't compile for Fesa.

The idea is to add an optional attribute to the silecs "Register" node, called "fesaValueItemName". If that attribute is present, it will be used in the codegen, if not, the name of the field will be used.