Improve namespace handling

[cleder]

Refactor to be independent of predefined namespaces and utilize the name_spaces attribute to determine the appropriate namespace.

[cleder]

To support namespaces on attributes as well, extend the fastkml.registry.registry.register method with an e.g. supported_nsids: Tuple[str, ..] parameter that loops over all namespaces for the registered functions (possibly as a decorator for the fastkml.helpers.*_kwargs functions)