feature: add new feature pkg to manage feature sets

This commit introduces a feature.Manager, which derives feature vectors
for various contexts within the daemon. The sets can be described via a
staticly compiled format, which makes any runtime adjustments to the
feature sets when the manager is initialized.

feature/default_sets.go

@@ -0,0 +1,32 @@
+package feature
+
+import "github.com/lightningnetwork/lnd/lnwire"
+
+// setDesc describes which feature bits should be advertised in which feature
+// sets.
+type setDesc map[lnwire.FeatureBit]map[Set]struct{}
+
+// defaultSetDesc are the default set descriptors for generating feature
+// vectors. Each set is annotated with the corresponding identifier from BOLT 9
+// indicating where it should be advertised.
+var defaultSetDesc = setDesc{
+	lnwire.DataLossProtectRequired: {
+		SetInit:    {}, // I
+		SetNodeAnn: {}, // N
+	},
+	lnwire.GossipQueriesOptional: {
+		SetInit:    {}, // I
+		SetNodeAnn: {}, // N
+	},
+	lnwire.TLVOnionPayloadOptional: {
+		SetInit:         {}, // I
+		SetNodeAnn:      {}, // N
+		SetInvoice:      {}, // 9
+		SetLegacyGlobal: {},
+	},
+	lnwire.StaticRemoteKeyOptional: {
+		SetInit:         {}, // I
+		SetNodeAnn:      {}, // N
+		SetLegacyGlobal: {},
+	},
+}