mtls refactor for endpoints

.changelog/4869.txt

@@ -0,0 +1,3 @@
+```release-note:enhancement
+added support for [mtls authentication](https://google.aip.dev/auth/4114) 
+```

google/mtls_util.go

@@ -0,0 +1,46 @@
+package google
+
+import (
+	"context"
+	"fmt"
+	"net/url"
+	"strings"
+
+	"google.golang.org/api/option/internaloption"
+	"google.golang.org/api/transport"
+)
+
+// The transport libaray does not natively expose logic to determine whether
+// the user is within mtls mode or not. They do return the mtls endpoint if
+// it is enabled during client creation so we will use this logic to determine
+// the mode the user is in and throw away the client they give us back.
+func isMtls() bool {
+	regularEndpoint := "https://mockservice.googleapis.com/v1/"
+	mtlsEndpoint := getMtlsEndpoint(regularEndpoint)
+	_, endpoint, err := transport.NewHTTPClient(context.Background(),
+		internaloption.WithDefaultEndpoint(regularEndpoint),
+		internaloption.WithDefaultMTLSEndpoint(mtlsEndpoint),
+	)
+	if err != nil {
+		return false
+	}
+	isMtls := endpoint == mtlsEndpoint
+	return isMtls
+}
+
+func getMtlsEndpoint(baseEndpoint string) string {
+	u, err := url.Parse(baseEndpoint)
+	if err != nil {
+		if strings.Contains(baseEndpoint, ".googleapis") {
+			return strings.Replace(baseEndpoint, ".googleapis", ".mtls.googleapis", 1)
+		}
+		return baseEndpoint
+	}
+	domainParts := strings.Split(u.Host, ".")
+	if len(domainParts) > 1 {
+		u.Host = fmt.Sprintf("%s.mtls.%s", domainParts[0], strings.Join(domainParts[1:], "."))
+	} else {
+		u.Host = fmt.Sprintf("%s.mtls", domainParts[0])
+	}
+	return u.String()
+}

google/mtls_util_test.go

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+package google
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestUnitMtls_urlSwitching(t *testing.T) {
+	t.Parallel()
+	for key, bp := range DefaultBasePaths {
+		url := getMtlsEndpoint(bp)
+		if !strings.Contains(url, ".mtls.") {
+			t.Errorf("%s: mtls conversion unsuccessful preconv - %s postconv - %s", key, bp, url)
+		}
+	}
+}