TUN-6772: Add a JWT Validator as an ingress verifier

This adds a new verifier interface that can be attached to ingress.Rule.
This would act as a middleware layer that gets executed at the start of
proxy.ProxyHTTP.

A jwt validator implementation for this verifier is also provided. The
validator downloads the public key from the access teams endpoint and
uses it to verify the JWT sent to cloudflared with the audtag (clientID)
information provided in the config.

vendor/google.golang.org/grpc/internal/metadata/metadata.go

@@ -22,6 +22,9 @@
 package metadata
 
 import (
+	"fmt"
+	"strings"
+
 	"google.golang.org/grpc/metadata"
 	"google.golang.org/grpc/resolver"
 )
@@ -72,3 +75,46 @@ func Set(addr resolver.Address, md metadata.MD) resolver.Address {
 	addr.Attributes = addr.Attributes.WithValue(mdKey, mdValue(md))
 	return addr
 }
+
+// Validate returns an error if the input md contains invalid keys or values.
+//
+// If the header is not a pseudo-header, the following items are checked:
+// - header names must contain one or more characters from this set [0-9 a-z _ - .].
+// - if the header-name ends with a "-bin" suffix, no validation of the header value is performed.
+// - otherwise, the header value must contain one or more characters from the set [%x20-%x7E].
+func Validate(md metadata.MD) error {
+	for k, vals := range md {
+		// pseudo-header will be ignored
+		if k[0] == ':' {
+			continue
+		}
+		// check key, for i that saving a conversion if not using for range
+		for i := 0; i < len(k); i++ {
+			r := k[i]
+			if !(r >= 'a' && r <= 'z') && !(r >= '0' && r <= '9') && r != '.' && r != '-' && r != '_' {
+				return fmt.Errorf("header key %q contains illegal characters not in [0-9a-z-_.]", k)
+			}
+		}
+		if strings.HasSuffix(k, "-bin") {
+			continue
+		}
+		// check value
+		for _, val := range vals {
+			if hasNotPrintable(val) {
+				return fmt.Errorf("header key %q contains value with non-printable ASCII characters", k)
+			}
+		}
+	}
+	return nil
+}
+
+// hasNotPrintable return true if msg contains any characters which are not in %x20-%x7E
+func hasNotPrintable(msg string) bool {
+	// for i that saving a conversion if not using for range
+	for i := 0; i < len(msg); i++ {
+		if msg[i] < 0x20 || msg[i] > 0x7E {
+			return true
+		}
+	}
+	return false
+}