clientcredentials: add option for additional endpoint parameters

This is to support https://auth0.com/docs/api-auth/config/asking-for-access-tokens.

Fixes https://github.com/golang/oauth2/issues/216

Change-Id: I9b8fdb4fe22c688fd71e43bd21d80b796434b8b0
Reviewed-on: https://go-review.googlesource.com/36880
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Jaana Burcu Dogan <jbd@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

clientcredentials/clientcredentials.go

@@ -14,6 +14,7 @@
 package clientcredentials // import "golang.org/x/oauth2/clientcredentials"
 
 import (
+	"fmt"
 	"net/http"
 	"net/url"
 	"strings"
@@ -38,6 +39,9 @@ type Config struct {
 
 	// Scope specifies optional requested permissions.
 	Scopes []string
+
+	// EndpointParams specifies additional parameters for requests to the token endpoint.
+	EndpointParams url.Values
 }
 
 // Token uses client credentials to retrieve a token.
@@ -76,10 +80,17 @@ type tokenSource struct {
 // Token refreshes the token by using a new client credentials request.
 // tokens received this way do not include a refresh token
 func (c *tokenSource) Token() (*oauth2.Token, error) {
-	tk, err := internal.RetrieveToken(c.ctx, c.conf.ClientID, c.conf.ClientSecret, c.conf.TokenURL, url.Values{
+	v := url.Values{
 		"grant_type": {"client_credentials"},
 		"scope":      internal.CondVal(strings.Join(c.conf.Scopes, " ")),
-	})
+	}
+	for k, p := range c.conf.EndpointParams {
+		if _, ok := v[k]; ok {
+			return nil, fmt.Errorf("oauth2: cannot overwrite parameter %q", k)
+		}
+		v[k] = p
+	}
+	tk, err := internal.RetrieveToken(c.ctx, c.conf.ClientID, c.conf.ClientSecret, c.conf.TokenURL, v)
 	if err != nil {
 		return nil, err
 	}