Currently when the OpenAIEmbeddingEncoder adds embeddings to Elements in
`_add_embeddings_to_elements` it overwrites each Element's `to_dict`
method, mistakenly resulting in each Element having identical values
with the exception of the actual embedding value. This was due to the
way it leverages a nested `new_to_dict` method to overwrite. Instead,
this updates the original definition of Element itself to accommodate
the `embeddings` field when available. This also adds a test to validate
that values are not duplicated.
