add an option to use a custom progress printer for the build progress output

[rarguelloF]

What I did

Adds a new optional field OutPrinter to the BuildOptions struct to allow customizing where the build progress goes. If this field is not specified, output will go to os.Stdout, which is the current behavior.

The motivation for this change is when building tooling around this project, you might want to capture these logs and use them accordingly, example:

type cacheMessagesPrinter struct {
	msgs []string
}

func (c *cacheMessagesPrinter) Write(p []byte) (n int, err error) {
	c.msgs = append(c.msgs, string(p))
	return len(p), nil
}

func TestMyStuff(t *testing.T) {
	myPrinter: = &cacheMessagesPrinter{}
	
	buildOpts := &api.BuildOptions{
		Pull:     true,
		Progress: "auto",
		Args:     nil,
		NoCache:  false,
		Quiet:    false,
		Memory:   0,
		OutPrinter: myPrinter,
	}
	err := startMyDockerComposeProject(buildOpts)
	if err != nil {
        // this is particularly useful if using t.Parallel(), so you can associate logs to the proper test instead of everything
        // being mixed in stdout.
		t.Log("docker compose project build failed - see build logs below")
		buildLogs := strings.Join(myPrinter.msgs, "\n")
		t.Log(buildLogs)
   }
}

Hope you find this feature useful, please let me know what you think!

Related issue

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