unstructured: add missing from_dict method

[anakin87]

from_dict method is missing but it is necessary to properly deserialize the API key.
For example, this fails:

import os
from haystack import default_from_dict
from haystack_integrations.components.converters.unstructured import UnstructuredFileConverter

os.environ["UNSTRUCTURED_API_KEY"] = "test"
conv = UnstructuredFileConverter()

ser_converter = conv.to_dict()

new_conv = default_from_dict(UnstructuredFileConverter, ser_converter)

error:

api_key_value = api_key.resolve_value() if api_key else None
AttributeError: 'dict' object has no attribute 'resolve_value'

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I added the missing from_dict method and a related test.