TUN-8807: Add support_datagram_v3 to remote feature rollout

Support rolling out the `support_datagram_v3` feature via remote feature rollout (DNS TXT record) with `dv3` key.

Consolidated some of the feature evaluation code into the features module to simplify the lookup of available features at runtime.

Reduced complexity for management logs feature lookup since it's a default feature.

Closes TUN-8807

features/features.go

@@ -12,7 +12,7 @@ const (
 )
 
 var (
-	DefaultFeatures = []string{
+	defaultFeatures = []string{
 		FeatureAllowRemoteConfig,
 		FeatureSerializedHeaders,
 		FeatureDatagramV2,
@@ -21,15 +21,30 @@ var (
 	}
 )
 
-func Contains(feature string) bool {
-	for _, f := range DefaultFeatures {
-		if f == feature {
-			return true
-		}
-	}
-	return false
+// Features set by user provided flags
+type staticFeatures struct {
+	PostQuantumMode *PostQuantumMode
 }
 
+type PostQuantumMode uint8
+
+const (
+	// Prefer post quantum, but fallback if connection cannot be established
+	PostQuantumPrefer PostQuantumMode = iota
+	// If the user passes the --post-quantum flag, we override
+	// CurvePreferences to only support hybrid post-quantum key agreements.
+	PostQuantumStrict
+)
+
+type DatagramVersion string
+
+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
+)
+
 // Remove any duplicates from the slice
 func Dedup(slice []string) []string {