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/header.go

@@ -0,0 +1,64 @@
+// Copyright 2023 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 (
+	"runtime"
+	"strings"
+	"unicode"
+)
+
+var (
+	// version is a package internal global variable for testing purposes.
+	version = runtime.Version
+)
+
+// versionUnknown is only used when the runtime version cannot be determined.
+const versionUnknown = "UNKNOWN"
+
+// goVersion returns a Go runtime version derived from the runtime environment
+// that is modified to be suitable for reporting in a header, meaning it has no
+// whitespace. If it is unable to determine the Go runtime version, it returns
+// versionUnknown.
+func goVersion() string {
+	const develPrefix = "devel +"
+
+	s := version()
+	if strings.HasPrefix(s, develPrefix) {
+		s = s[len(develPrefix):]
+		if p := strings.IndexFunc(s, unicode.IsSpace); p >= 0 {
+			s = s[:p]
+		}
+		return s
+	} else if p := strings.IndexFunc(s, unicode.IsSpace); p >= 0 {
+		s = s[:p]
+	}
+
+	notSemverRune := func(r rune) bool {
+		return !strings.ContainsRune("0123456789.", r)
+	}
+
+	if strings.HasPrefix(s, "go1") {
+		s = s[2:]
+		var prerelease string
+		if p := strings.IndexFunc(s, notSemverRune); p >= 0 {
+			s, prerelease = s[:p], s[p:]
+		}
+		if strings.HasSuffix(s, ".") {
+			s += "0"
+		} else if strings.Count(s, ".") < 2 {
+			s += ".0"
+		}
+		if prerelease != "" {
+			// Some release candidates already have a dash in them.
+			if !strings.HasPrefix(prerelease, "-") {
+				prerelease = "-" + prerelease
+			}
+			s += prerelease
+		}
+		return s
+	}
+	return "UNKNOWN"
+}