Add support from streaming build logs from the Function Builder

[welteki]

Description

This change adds a new method BuildWithStream to the function builder
client that returns a BuildResultStream which can be used to iterate
over build results in real time as the build progresses.

How has this been tested:

Unit test were added.

Verified E2E using the example described in the docs:

// Invoke the function builder with the tar archive containing the build config and context
// to build and push the function image.
stream, err := b.BuildWithStream(tarPath)
if err != nil {
	log.Fatal(err)
}
defer stream.Close()

for event, err := range stream.Results() {
	if err != nil {
		log.Fatal(err)
	}

	if event.Log != nil {
		for _, logMsg := range event.Log {
			fmt.Printf("%s\n", logMsg)
		}
	}

	if event.Status == "success" || event.Status == "failed" {
		fmt.Printf("Status: %s\n", event.Status)
		fmt.Printf("Image: %s\n", event.Image)

		if len(event.Error) > 0 {
			fmt.Printf("Error: %s\n", event.Error)
		}
	}
}

[derek]

Thank you for your contribution. unfortunately, one or more of your commits are missing the required "Signed-off-by:" statement. Signing off is part of the Developer Certificate of Origin (DCO) which is used by this project.

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