lnwire: add type CustomRecords

This commit introduces the `CustomRecords` type in the `lnwire` package,
designed to hold arbitrary byte slices. Each entry in this map can
associate with TLV type values that are greater than or equal to 65536.

lnwire/encoding.go

@@ -1,5 +1,7 @@
 package lnwire
 
+import "github.com/lightningnetwork/lnd/tlv"
+
 // QueryEncoding is an enum-like type that represents exactly how a set data is
 // encoded on the wire.
 type QueryEncoding uint8
@@ -15,3 +17,17 @@ const (
 	// NOTE: this should no longer be used or accepted.
 	EncodingSortedZlib QueryEncoding = 1
 )
+
+// recordProducer is a simple helper struct that implements the
+// tlv.RecordProducer interface.
+type recordProducer struct {
+	record tlv.Record
+}
+
+// Record returns the underlying record.
+func (r *recordProducer) Record() tlv.Record {
+	return r.record
+}
+
+// Ensure that recordProducer implements the tlv.RecordProducer interface.
+var _ tlv.RecordProducer = (*recordProducer)(nil)