tailcfg: move NodeKey type to types/key.

This leaves behind a type alias and associated constructor, to allow
for gradual switchover.

Updates #3206.

Signed-off-by: David Anderson <danderson@tailscale.com>

types/key/node.go

@@ -307,3 +307,10 @@ func (k NodePublic) WireGuardGoString() string {
 	b[second+3] = b64((k.k[31] << 2) & 63)
 	return string(b)
 }
+
+// AsNodeKey returns k converted to a NodeKey.
+//
+// Cross-compatibility shim as part of #3206.
+func (k NodePublic) AsNodeKey() NodeKey {
+	return k.Raw32()
+}