TUN-9291: Remove dynamic reloading of features for datagram v3

During a refresh of the supported features via the DNS TXT record,
cloudflared would update the internal feature list, but would not
propagate this information to the edge during a new connection.

This meant that a situation could occur in which cloudflared would
think that the client's connection could support datagram V3, and
would setup that muxer locally, but would not propagate that information
to the edge during a register connection in the `ClientInfo` of the
`ConnectionOptions`. This meant that the edge still thought that the
client was setup to support datagram V2 and since the protocols are
not backwards compatible, the local muxer for datagram V3 would reject
the incoming RPC calls.

To address this, the feature list will be fetched only once during
client bootstrapping and will persist as-is until the client is restarted.
This helps reduce the complexity involved with different connections
having possibly different sets of features when connecting to the edge.
The features will now be tied to the client and never diverge across
connections.

Also, retires the use of `support_datagram_v3` in-favor of
`support_datagram_v3_1` to reduce the risk of reusing the feature key.
The `dv3` TXT feature key is also deprecated.

Closes TUN-9291

features/features.go

@@ -1,5 +1,7 @@
 package features
 
+import "slices"
+
 const (
 	FeatureSerializedHeaders = "serialized_headers"
 	FeatureQuickReconnects   = "quick_reconnects"
@@ -8,7 +10,9 @@ const (
 	FeaturePostQuantum       = "postquantum"
 	FeatureQUICSupportEOF    = "support_quic_eof"
 	FeatureManagementLogs    = "management_logs"
-	FeatureDatagramV3        = "support_datagram_v3"
+	FeatureDatagramV3_1      = "support_datagram_v3_1"
+
+	DeprecatedFeatureDatagramV3 = "support_datagram_v3" // Deprecated: TUN-9291
 )
 
 var defaultFeatures = []string{
@@ -19,6 +23,11 @@ var defaultFeatures = []string{
 	FeatureManagementLogs,
 }
 
+// List of features that are no longer in-use.
+var deprecatedFeatures = []string{
+	DeprecatedFeatureDatagramV3,
+}
+
 // Features set by user provided flags
 type staticFeatures struct {
 	PostQuantumMode *PostQuantumMode
@@ -40,15 +49,19 @@ const (
 	// DatagramV2 is the currently supported datagram protocol for UDP and ICMP packets
 	DatagramV2 DatagramVersion = FeatureDatagramV2
 	// DatagramV3 is a new datagram protocol for UDP and ICMP packets. It is not backwards compatible with datagram v2.
-	DatagramV3 DatagramVersion = FeatureDatagramV3
+	DatagramV3 DatagramVersion = FeatureDatagramV3_1
 )
 
-// Remove any duplicates from the slice
-func Dedup(slice []string) []string {
+// Remove any duplicate features from the list and remove deprecated features
+func dedupAndRemoveFeatures(features []string) []string {
 	// Convert the slice into a set
-	set := make(map[string]bool, 0)
-	for _, str := range slice {
-		set[str] = true
+	set := map[string]bool{}
+	for _, feature := range features {
+		// Remove deprecated features from the provided list
+		if slices.Contains(deprecatedFeatures, feature) {
+			continue
+		}
+		set[feature] = true
 	}
 
 	// Convert the set back into a slice