fs: change Config callback into state based callback #3455

This is a very large change which turns the post Config function in
backends into a state based call and response system so that
alternative user interfaces can be added.

The existing config logic has been converted, but it is quite
complicated and folloup commits will likely be needed to fix it!

Follow up commits will add a command line and API based way of using
this configuration system.

backend/googlephotos/googlephotos.go

@@ -77,36 +77,36 @@ func init() {
 		Prefix:      "gphotos",
 		Description: "Google Photos",
 		NewFs:       NewFs,
-		Config: func(ctx context.Context, name string, m configmap.Mapper) error {
+		Config: func(ctx context.Context, name string, m configmap.Mapper, config fs.ConfigIn) (*fs.ConfigOut, error) {
 			// Parse config into Options struct
 			opt := new(Options)
 			err := configstruct.Set(m, opt)
 			if err != nil {
-				return errors.Wrap(err, "couldn't parse config into struct")
+				return nil, errors.Wrap(err, "couldn't parse config into struct")
 			}
 
-			// Fill in the scopes
-			if opt.ReadOnly {
-				oauthConfig.Scopes[0] = scopeReadOnly
-			} else {
-				oauthConfig.Scopes[0] = scopeReadWrite
+			switch config.State {
+			case "":
+				// Fill in the scopes
+				if opt.ReadOnly {
+					oauthConfig.Scopes[0] = scopeReadOnly
+				} else {
+					oauthConfig.Scopes[0] = scopeReadWrite
+				}
+				return oauthutil.ConfigOut("warning", &oauthutil.Options{
+					OAuth2Config: oauthConfig,
+				})
+			case "warning":
+				// Warn the user as required by google photos integration
+				return fs.ConfigConfirm("warning_done", true, `Warning
+
+IMPORTANT: All media items uploaded to Google Photos with rclone
+are stored in full resolution at original quality.  These uploads
+will count towards storage in your Google Account.`)
+			case "warning_done":
+				return nil, nil
 			}
-
-			// Do the oauth
-			err = oauthutil.Config(ctx, "google photos", name, m, oauthConfig, nil)
-			if err != nil {
-				return errors.Wrap(err, "failed to configure token")
-			}
-
-			// Warn the user
-			fmt.Print(`
-*** IMPORTANT: All media items uploaded to Google Photos with rclone
-*** are stored in full resolution at original quality.  These uploads
-*** will count towards storage in your Google Account.
-
-`)
-
-			return nil
+			return nil, fmt.Errorf("unknown state %q", config.State)
 		},
 		Options: append(oauthutil.SharedOptions, []fs.Option{{
 			Name:    "read_only",