Add support for storing charts in GCS.

[kylieCat]

• Charts can be in public or private GCS buckets. Uses APPLICATION_DEFAULT_CREDENTIALS to authenticate
• Usage:

chart_options = {
    "name":"my-chart",
    "source": {
    	"type": "repo",
        "location": f"gs://{bucket_name}"

     }
}

chart = ChartBuilder(chart_options)

[yanivoliver]

Hi @kylie-a - the code looks good, but please add tests for the added functionality.

[yanivoliver]

You didn't actually mock mocked_gcsclient in the tests. They do not pass.