Allow users to provide their own storage client

[emilong]

I had a few other bits of configuration I wanted to do on my Google::Cloud::Storage client instance beyond project and credentials, so I thought I'd just allow passing that in rather than adding more options.

Happy to discuss other alternatives though!

[4ndypanda]

+1 to supporting more options to pass to Google::Cloud::Storage. In our repo, we created a subclass of Shrine::Storage::GoogleCloudStorage and overrode the method #storage to be able to pass it more configs

  # @override
  def storage
    return @storage if @storage

    opts = {}
    opts[:project] = @project if @project
    opts[:credentials] = @credentials if @credentials
    # @storage_options initialized in our constructor
    opts.merge!(@storage_options)

    @storage = Google::Cloud::Storage.new(**opts)
  end