Simplify GetOutput

[tri-adam]

GetOutput currently accepts an type OutputReader interface, which the caller must implement:

scs-build-client/client/output.go

Lines 18 to 22 in 3907a92

	// OutputReader interface is used to read the websocket output from the stream
	type OutputReader interface {
	// Read is called when a websocket message is received
	Read(messageType int, p []byte) (int, error)
	}

The messageType int here is problematic, because it requires the caller to understand implementation details. In particular, the caller needs to understand the possible messageType values, and how to handle them. In practice, these are defined as "text" and "binary" in RFC 6455. All implementations that I could find handle text messages, and discard binary ones.

With all that in mind, I propose we change the signature from this:

func (c *Client) GetOutput(ctx context.Context, buildID string, or OutputReader) error

To this:

func (c *Client) GetOutput(ctx context.Context, buildID string, w io.Writer) error

This way, the caller doesn't need to worry about any of the web socket implementation details under the hood, and can pass any io.Writer such as os.Stdout, simplifying usage.