Adds GCS snippets, was blocked by https://goo.gl/uWgYhQ

[gguuss]

This is a boilerplate change, for GCS, we construct the image used in the request as:

    ImageSource imgSource = ImageSource.newBuilder().setGcsImageUri(gcsPath).build();
    Image img = Image.newBuilder().setSource(imgSource).build();

instead of:

    ByteString imgBytes = ByteString.readFrom(new FileInputStream(filePath));
    Image img = Image.newBuilder().setContent(imgBytes).build();

Tests were also added for the 7 new functions.