google/externalaccount: moves externalaccount package out of internal and exports it

go/programmable-auth-design for context. Adds support for user defined
 supplier methods to return subject tokens and AWS security credentials.

Change-Id: I7bc41f8c5202ae933fce516632f5049bbeb3d378
GitHub-Last-Rev: ac519b242f8315df572f1b205b0670f139bfc6c3
GitHub-Pull-Request: golang/oauth2#690
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/550835
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Leo Siracusa <leosiracusa@google.com>
Reviewed-by: Chris Smith <chrisdsmith@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cody Oss <codyoss@google.com>

google/externalaccount/programmaticrefreshcredsource.go

@@ -0,0 +1,21 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package externalaccount
+
+import "context"
+
+type programmaticRefreshCredentialSource struct {
+	supplierOptions      SupplierOptions
+	subjectTokenSupplier SubjectTokenSupplier
+	ctx                  context.Context
+}
+
+func (cs programmaticRefreshCredentialSource) credentialSourceType() string {
+	return "programmatic"
+}
+
+func (cs programmaticRefreshCredentialSource) subjectToken() (string, error) {
+	return cs.subjectTokenSupplier.SubjectToken(cs.ctx, cs.supplierOptions)
+}