oauth2: auto-detect auth style by default, add Endpoint.AuthStyle

Instead of maintaining a global map of which OAuth2 servers do which
auth style and/or requiring the user to tell us, just try both ways
and remember which way worked. But if users want to tell us in the
Endpoint, this CL also add Endpoint.AuthStyle.

Fixes golang/oauth2#111
Fixes golang/oauth2#365
Fixes golang/oauth2#362
Fixes golang/oauth2#357
Fixes golang/oauth2#353
Fixes golang/oauth2#345
Fixes golang/oauth2#326
Fixes golang/oauth2#352
Fixes golang/oauth2#268
Fixes https://go-review.googlesource.com/c/oauth2/+/58510
(... and surely many more ...)

Change-Id: I7b4d98ba1900ee2d3e11e629316b0bf867f7d237
Reviewed-on: https://go-review.googlesource.com/c/157820
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ross Light <light@google.com>

clientcredentials/clientcredentials.go

@@ -42,6 +42,11 @@ type Config struct {
 
 	// EndpointParams specifies additional parameters for requests to the token endpoint.
 	EndpointParams url.Values
+
+	// AuthStyle optionally specifies how the endpoint wants the
+	// client ID & client secret sent. The zero value means to
+	// auto-detect.
+	AuthStyle oauth2.AuthStyle
 }
 
 // Token uses client credentials to retrieve a token.
@@ -97,7 +102,8 @@ func (c *tokenSource) Token() (*oauth2.Token, error) {
 		}
 		v[k] = p
 	}
-	tk, err := internal.RetrieveToken(c.ctx, c.conf.ClientID, c.conf.ClientSecret, c.conf.TokenURL, v)
+
+	tk, err := internal.RetrieveToken(c.ctx, c.conf.ClientID, c.conf.ClientSecret, c.conf.TokenURL, v, internal.AuthStyle(c.conf.AuthStyle))
 	if err != nil {
 		if rErr, ok := err.(*internal.RetrieveError); ok {
 			return nil, (*oauth2.RetrieveError)(rErr)