config package rework (#9)

* change config interface
* reuse codec
* allow to modify next config based on values in current config via BeforeLoad

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

client/lookup.go

@@ -2,6 +2,7 @@ package client
 
 import (
 	"context"
+	"fmt"
 	"sort"
 
 	"github.com/unistack-org/micro/v3/errors"
@@ -18,6 +19,10 @@ func LookupRoute(ctx context.Context, req Request, opts CallOptions) ([]string,
 		return opts.Address, nil
 	}
 
+	if opts.Router == nil {
+		return nil, router.ErrRouteNotFound
+	}
+
 	// construct the router query
 	query := []router.QueryOption{router.QueryService(req.Service())}