Allow mapping prefixes to be tripper.Namespace objects

[jesper-friis]

We would like to be able to configure mappings as follows:

mapping1 = client.create_mapping(
    mappingType="triples",
    prefixes={
        "emmo": EMMO,
        "map": MAP,
        "cif": CIF,
        "structure": STRUCTURE,
        "energy": ENERGY,
        "forces": FORCES,
    },
    triples = [
        (STRUCTURE.positions, MAP.mapsTo, EMMO.PositionVector),
        (STRUCTURE.symbols,   MAP.mapsTo, EMMO.Symbol),
        (STRUCTURE.cell,      MAP.mapsTo, CIF.cell),
    ]
)

where EMMO, MAP, ... are tripper namespaces. However, the MappingConfig pydantic model requires us to write

mapping1 = client.create_mapping(
    mappingType="triples",
    prefixes={
        "emmo": str(EMMO),
        "map": str(MAP),
        "cif": str(CIF),
        "structure": str(STRUCTURE),
        "energy": str(ENERGY),
        "forces": str(FORCES),
    },
    triples = [
        (STRUCTURE.positions, MAP.mapsTo, EMMO.PositionVector),
        (STRUCTURE.symbols,   MAP.mapsTo, EMMO.Symbol),
        (STRUCTURE.cell,      MAP.mapsTo, CIF.cell),
    ]
)

which is not that nice.