Use optional headers in DownloadableFile messages when fetching files

[tpaschalis]

A recent spec change enables the DownloadableFile message to set optional headers to use when downloading a file, for example to set up some authentication headers.

The change should be straightforward; the code path which downloads files ultimately ends up here:

opamp-go/client/internal/packagessyncer.go

Lines 274 to 301 in b33ab76

	// downloadFile downloads the file from the server.
	func (s *packagesSyncer) downloadFile(ctx context.Context, pkgName string, file *protobufs.DownloadableFile) error {
	s.logger.Debugf(ctx, "Downloading package %s file from %s", pkgName, file.DownloadUrl)
	req, err := http.NewRequestWithContext(ctx, "GET", file.DownloadUrl, nil)
	if err != nil {
	return fmt.Errorf("cannot download file from %s: %v", file.DownloadUrl, err)
	}
	resp, err := http.DefaultClient.Do(req)
	if err != nil {
	return fmt.Errorf("cannot download file from %s: %v", file.DownloadUrl, err)
	}
	defer func() { _ = resp.Body.Close() }()
	if resp.StatusCode != http.StatusOK {
	return fmt.Errorf("cannot download file from %s, HTTP response=%v", file.DownloadUrl, resp.StatusCode)
	}
	// TODO: either add a callback to verify file.Signature or pass the Signature
	// as a parameter to UpdateContent.
	err = s.localState.UpdateContent(ctx, pkgName, resp.Body, file.ContentHash)
	if err != nil {
	return fmt.Errorf("failed to install/update the package %s downloaded from %s: %v", pkgName, file.DownloadUrl, err)
	}
	return nil
	}