use google vision explicit authentication

[10zinten]

Goal: To have explicit google vision authentication for every pipeline run

• [ ] change this line https://github.com/OpenPecha/OCR-Pipelines/blob/54f53340c80ab7ca29ae37c274a6883e8066af5f/ocr_pipelines/executor.py#L30 to the following

credentials = google.oauth2.credential.Credential()
self.vision_client = vision.ImageAnnotatorClient(credentials=credentials)

- expose credential to `__init__`
def __init__(self, images_base_dir: Path, config: ImportConfig, ocr_base_dir: Path, credential: Dict):
     ...