Integration with the Google Cloud Vision API (#2902)
This PR adds a third OCR provider, alongside Tesseract and Paddle: the
[Google Cloud Vision API](https://cloud.google.com/vision).
It can be used similarly to other OCR methods: set the `OCR_AGENT`
environment variable to the path to the OCR module
(`unstructured.partition.utils.ocr_models.google_vision_ocr.OCRAgentGoogleVision`).
You also need to set the credentials to use Google APIs, for instance by
setting the `GOOGLE_APPLICATION_CREDENTIALS` environment variable.
---------
Co-authored-by: christinestraub <christinemstraub@gmail.com>
