multi: add base lookup option to AddLocalAlias

We add an extra option to the AddLocalAlias method which only controls
whether we store a reverse lookup from the alias back to the base scid
it corresponds to. The previous flag "gossip" is still maintained, and
in a way supercedes the new flag (it will also store the base scid
lookup even if the base lookup flag isn't set). The only call that sets
this option is the XAddLocalChanAlias RPC endpoint, where we want to
make sure that a reverse lookup is stored in the alias manager in order
to later expose it via the new RPC method.

lnrpc/routerrpc/router_server.go

@@ -1713,8 +1713,13 @@ func (s *Server) XAddLocalChanAliases(_ context.Context,
 					rpcAlias)
 			}
 
+			// We set the baseLookup flag as we want the alias
+			// manager to keep a mapping from the alias back to its
+			// base scid, in order to be able to provide it via the
+			// FindBaseLocalChanAlias RPC.
 			err = s.cfg.AliasMgr.AddLocalAlias(
 				aliasScid, baseScid, false, true,
+				aliasmgr.WithBaseLookup(),
 			)
 			if err != nil {
 				return nil, fmt.Errorf("error adding scid "+