Add support for GCS Server objects

[hcrosse]

Create a GCS server object similar to the S3 one.

It's possible this is unnecessary due to GCS's S3 interoperability, in which case my ask changes to an update to the documentation to make this use explicitly stated (which I'll copy over to the specification repo) in the README.

[jochenchrist]

Let's stick with the s3 config option (and its endpointUrl option) and add a documentation for GCS, Azure Blob Storage, ... otherwise we will come with an explosion of similar file storage configs.

[hcrosse]

It looks like there's already an Azure block so if the rest of the CLI already supports GCS using the S3 endpoint we just need to add this

[jochenchrist]

OK, let's add explicit support for server type gcs.

[hcrosse]

Closed by https://github.com/datacontract/datacontract-cli/commit/17b291299b6df59b34963d299f0299913de7419a