Expose ability to set blob metadata in GCSHook

Description

Expose ability to set blob metadata in the GCSHook. Probably via the upload method with a new parameter: metadata: Optional[Dict[str, str]] = None.

Use case / motivation

As a best practice, I always set blob metadata attributes which are useful or provide more information. Any information about the blob, such as tracking information, foreign keys, job information, etc.

Using the Google SDK, this is what it looks like:

metadata = {
    "job-id": job_id,
    "user-id": user_id,
    "batch-size": batch_size,
    # ...
}

storage_client = storage.Client()
bucket = storage_client.bucket(gcp_bucket_name)
blob = bucket.blob(destination_blob_name)
blob.content_type = content_type
blob.metadata = metadata

[potiuk]

And i believe you can make it in non-breaking way. Airflow requires all the operator init args.to be keyword args, so adding a new keyword with default should not be breaking

[potiuk]

Airflow is community driven, so it is normal for users like you to contribute. Many do. It’s a nice way to give back to the community.

See CONTRIBITING.rst in the root of the repo (the repo is exactly the one we are discussing it).