Fns now annotated with invoke urls, as per triggers (#1172)

Clone of the trigger work to inject invoke urls into the annotations
on a fn when it is returned from the server.

Small changes to trigges code following code review of the fn code.

api/server/fns_get.go

@@ -1,6 +1,7 @@
 package server
 
 import (
+	"fmt"
 	"net/http"
 
 	"github.com/fnproject/fn/api"
@@ -9,11 +10,24 @@ import (
 
 func (s *Server) handleFnGet(c *gin.Context) {
 	ctx := c.Request.Context()
+
 	f, err := s.datastore.GetFnByID(ctx, c.Param(api.ParamFnID))
 	if err != nil {
 		handleErrorResponse(c, err)
 		return
 	}
 
+	app, err := s.datastore.GetAppByID(ctx, f.AppID)
+	if err != nil {
+		handleErrorResponse(c, fmt.Errorf("unexpected error - fn app not available: %s", err))
+		return
+	}
+
+	f, err = s.fnAnnotator.AnnotateFn(c, app, f)
+	if err != nil {
+		handleErrorResponse(c, err)
+		return
+	}
+
 	c.JSON(http.StatusOK, f)
 }