feat: Add support for TEI API key authentication

[kenwoodjw]

Summary

Fixes #9112

• Introduce api_key parameter in the TeiHelper class to enable API key-based authentication.
• Dynamically include the Authorization: Bearer {api_key} header in HTTP requests for endpoints such as tokenize, embeddings, and rerank.

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Checklist

Important

Please review the checklist below before submitting your pull request.

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods