Support options for handlers & refactor geoflow handler fun args

[eblondel]

The possibility to configure and read options for handlers (for entities, contacts, dictionary) will be added. Together with this, the loadMetadataHandler will be refactored as enhancement to return the object of class geoflow_handler, and the function associated with an handler will be refactored:

• previously: the args of the function were config, source
• next: the args of the function will be handler, source, config, by analogy with the action fun modelling (see Refactor geoflow action fun args #262) that handles args action, entity, config

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