feat: add multimodal(mm).exif function

[GarrettWu]

The function deployment needs connections ${BQ_LOCATION}.multimodal-udf-connection to be created, which is used for Python udf to access networks. The connections need to be shared publicly for callers to use. No connection SA permissions needed.

To run the tests, another connection ${BQ_LOCATION}.bigframes-default-connection is needed. Which is a user provided connection to create ObjectRef and access GCS files. storage.ObjectUser role to the GCS bucket is needed for the connection SA. It's user data, nothing related to the function.

I'm reusing the JS_BUCKET for test data. Since there need to be a user bucket to upload and run test against. I feel specifying another bucket is redundant. But now the variable name isn't accurate. Lemme know if we want to rename the variable or need another bucket.

The function "exif" is the one we want users to call. "exif_udf" is a helper, so I didn't include it in the README.

Tests are passing locally with bigframes-dev project. screen/5ZNDCSCfrkURoT8