Add default user-agent header

[alvarobartt]

What does this PR do?

This PR simply adds the text-embeddings-inference/<version> value in the user-agent header to track downloads of models from the Hugging Face Hub with origin in this repository.

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• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you make sure to update the documentation with your changes? Here are the documentation guidelines.
• Did you write any new necessary tests? If applicable, did you include or update the insta snapshots?

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