Honor DurationSeconds properly for WebIdentity (#8581)

Also cleanup code to add various constants for
verbatim strings across the code base.

Fixes #8482

pkg/auth/credentials_test.go

@@ -16,7 +16,43 @@
 
 package auth
 
-import "testing"
+import (
+	"encoding/json"
+	"testing"
+	"time"
+)
+
+func TestExpToInt64(t *testing.T) {
+	testCases := []struct {
+		exp             interface{}
+		expectedFailure bool
+	}{
+		{"", true},
+		{"-1", true},
+		{"1574812326", false},
+		{1574812326, false},
+		{int64(1574812326), false},
+		{int(1574812326), false},
+		{uint(1574812326), false},
+		{uint64(1574812326), false},
+		{json.Number("1574812326"), false},
+		{1574812326.000, false},
+		{time.Duration(3) * time.Minute, false},
+	}
+
+	for _, testCase := range testCases {
+		testCase := testCase
+		t.Run("", func(t *testing.T) {
+			_, err := ExpToInt64(testCase.exp)
+			if err != nil && !testCase.expectedFailure {
+				t.Errorf("Expected success but got failure %s", err)
+			}
+			if err == nil && testCase.expectedFailure {
+				t.Error("Expected failure but got success")
+			}
+		})
+	}
+}
 
 func TestIsAccessKeyValid(t *testing.T) {
 	testCases := []struct {