lib/oauthutil: add support for OAuth client credential flow

This commit reorganises the oauth code to use our own config struct
which has all the info for the normal oauth method and also the client
credentials flow method.

It updates all backends which use lib/oauthutil to use the new config
struct which shouldn't change any functionality.

It also adds code for dealing with the client credential flow config
which doesn't require the use of a browser and doesn't have or need a
refresh token.

Co-authored-by: Nick Craig-Wood <nick@craig-wood.com>

backend/putio/putio.go

@@ -13,7 +13,6 @@ import (
 	"github.com/rclone/rclone/lib/dircache"
 	"github.com/rclone/rclone/lib/encoder"
 	"github.com/rclone/rclone/lib/oauthutil"
-	"golang.org/x/oauth2"
 )
 
 /*
@@ -41,12 +40,10 @@ const (
 
 var (
 	// Description of how to auth for this app
-	putioConfig = &oauth2.Config{
-		Scopes: []string{},
-		Endpoint: oauth2.Endpoint{
-			AuthURL:  "https://api.put.io/v2/oauth2/authenticate",
-			TokenURL: "https://api.put.io/v2/oauth2/access_token",
-		},
+	putioConfig = &oauthutil.Config{
+		Scopes:       []string{},
+		AuthURL:      "https://api.put.io/v2/oauth2/authenticate",
+		TokenURL:     "https://api.put.io/v2/oauth2/access_token",
 		ClientID:     rcloneClientID,
 		ClientSecret: obscure.MustReveal(rcloneObscuredClientSecret),
 		RedirectURL:  oauthutil.RedirectLocalhostURL,