Make GCS support optional

[adriangonz]

Context

We currently rely on KFServing's Storage.py implementation, which adds support for Azure, GCS, S3 and local file downloading. Each of these requires its own library (e.g. google-cloud-storage, minio, etc.) However, on most cases we won't need support for all of them at the same time. Therefore, it makes sense to make some of them optional, so that seldon_core doesn't get bloated.

A good starting point for this would be making google-cloud-storage optional.

Suggested fix

In a similar way as we did for tensorflow, we could add google-cloud-storage as an optional dependency, so that it gets installed as:

$ pip install seldon_core[gcs]

On the long term, it could be good looking at libraries which offer support for the main cloud providers so that we don't need to import all of these.