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/sharefile/sharefile.go

@@ -97,7 +97,6 @@ import (
 	"github.com/rclone/rclone/lib/pacer"
 	"github.com/rclone/rclone/lib/random"
 	"github.com/rclone/rclone/lib/rest"
-	"golang.org/x/oauth2"
 )
 
 const (
@@ -115,13 +114,11 @@ const (
 )
 
 // Generate a new oauth2 config which we will update when we know the TokenURL
-func newOauthConfig(tokenURL string) *oauth2.Config {
-	return &oauth2.Config{
-		Scopes: nil,
-		Endpoint: oauth2.Endpoint{
-			AuthURL:  "https://secure.sharefile.com/oauth/authorize",
-			TokenURL: tokenURL,
-		},
+func newOauthConfig(tokenURL string) *oauthutil.Config {
+	return &oauthutil.Config{
+		Scopes:       nil,
+		AuthURL:      "https://secure.sharefile.com/oauth/authorize",
+		TokenURL:     tokenURL,
 		ClientID:     rcloneClientID,
 		ClientSecret: obscure.MustReveal(rcloneEncryptedClientSecret),
 		RedirectURL:  oauthutil.RedirectPublicSecureURL,
@@ -136,7 +133,7 @@ func init() {
 		NewFs:       NewFs,
 		Config: func(ctx context.Context, name string, m configmap.Mapper, config fs.ConfigIn) (*fs.ConfigOut, error) {
 			oauthConfig := newOauthConfig("")
-			checkAuth := func(oauthConfig *oauth2.Config, auth *oauthutil.AuthResult) error {
+			checkAuth := func(oauthConfig *oauthutil.Config, auth *oauthutil.AuthResult) error {
 				if auth == nil || auth.Form == nil {
 					return errors.New("endpoint not found in response")
 				}
@@ -147,7 +144,7 @@ func init() {
 				}
 				endpoint := "https://" + subdomain + "." + apicp
 				m.Set("endpoint", endpoint)
-				oauthConfig.Endpoint.TokenURL = endpoint + tokenPath
+				oauthConfig.TokenURL = endpoint + tokenPath
 				return nil
 			}
 			return oauthutil.ConfigOut("", &oauthutil.Options{