authhandler: Add support for PKCE

- Added new TokenSourceWithPKCE function to authhandler package.
- Updated Token method to support PKCE flow, sending code challenge and challenge method on the auth-code request, and sending code verifier on the exchange request.
- Updated google/default.go to support PKCE param.

Change-Id: Iab895bc01407c4742706061753f5329a772068ec
GitHub-Last-Rev: c1fddd28bcc4c7bd265573111808427711d7ba37
GitHub-Pull-Request: golang/oauth2#568
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/410515
Run-TryBot: Cody Oss <codyoss@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Shin Fan <shinfan@google.com>
Reviewed-by: Cody Oss <codyoss@google.com>

google/default.go

@@ -54,11 +54,14 @@ type CredentialsParams struct {
 	// Optional.
 	Subject string
 
-	// AuthHandler is the AuthorizationHandler used for 3-legged OAuth flow. Optional.
+	// AuthHandler is the AuthorizationHandler used for 3-legged OAuth flow. Required for 3LO flow.
 	AuthHandler authhandler.AuthorizationHandler
 
-	// State is a unique string used with AuthHandler. Optional.
+	// State is a unique string used with AuthHandler. Required for 3LO flow.
 	State string
+
+	// PKCE is used to support PKCE flow. Optional for 3LO flow.
+	PKCE *authhandler.PKCEParams
 }
 
 func (params CredentialsParams) deepCopy() CredentialsParams {
@@ -176,7 +179,7 @@ func CredentialsFromJSONWithParams(ctx context.Context, jsonData []byte, params
 	if config != nil {
 		return &Credentials{
 			ProjectID:   "",
-			TokenSource: authhandler.TokenSource(ctx, config, params.State, params.AuthHandler),
+			TokenSource: authhandler.TokenSourceWithPKCE(ctx, config, params.State, params.AuthHandler, params.PKCE),
 			JSON:        jsonData,
 		}, nil
 	}