identity: support jwt expiration and rotation (#18262)

Implements expirations and renewals for alternate workload identity tokens.

testutil/file.go

@@ -0,0 +1,20 @@
+// Copyright (c) HashiCorp, Inc.
+// SPDX-License-Identifier: BUSL-1.1
+
+package testutil
+
+import (
+	"os"
+	"path/filepath"
+
+	testing "github.com/mitchellh/go-testing-interface"
+	"github.com/shoenig/test/must"
+)
+
+// MustReadFile returns the contents of the specified file or fails the test.
+// Multiple arguments are joined with filepath.Join.
+func MustReadFile(t testing.T, path ...string) []byte {
+	contents, err := os.ReadFile(filepath.Join(path...))
+	must.NoError(t, err)
+	return contents
+}