(De-) Serialization is not properly working for NamedEntityExtractor

[julian-risch]

Describe the bug
Similar to #7660

When a pipeline containing a NamedEntityExtractor is deserialized, the deserialization fails with:

ConstructorError(None, None,
yaml.constructor.ConstructorError: could not determine a constructor for the tag 'tag:yaml.org,2002:python/object/apply:haystack.components.extractors.named_entity_extractor.NamedEntityExtractorBackend'
  in "<unicode string>", line 4, column 16:
          backend: !!python/object/apply:haystack.c ...

This is caused by the NamedEntityExtractorBackend enum.

To Reproduce

from haystack import Pipeline
from haystack.components.extractors import NamedEntityExtractor, NamedEntityExtractorBackend
extractor = NamedEntityExtractor(backend=NamedEntityExtractorBackend.HUGGING_FACE, model="dslim/bert-base-NER")

p = Pipeline()
p.add_component(instance=extractor, name="extractor")

with open("test_pipeline.yaml", "w") as f:
    p.dump(f)
with open("test_pipeline.yaml", "r") as f:
    q = Pipeline.load(f)