Deprecate native_namespace in favour of backend in IO functions

[MarcoGorelli]

native_namespace is annoyingly long, and retrieving native namespaces can be tedious

I think from_dict (and other IO functions) could have a backend: ModuleType | Implementation | EagerBackend argument, so all of the following could be supported:

data = {'a': [1,2,3], 'b': [4,5,6]}

nw.from_dict(data, backend=pyarrow)
nw.from_dict(data, backend='pyarrow')
nw.from_dict(data, backend=nw.Implementation.PYARROW)

EagerBackend could be Literal['pandas', 'polars', 'pyarrow']