Add support for Function and Trigger domain objects (#1060)

Vast commit, includes:

 * Introduces the Trigger domain entity.
 * Introduces the Fns domain entity.
 * V2 of the API for interacting with the new entities in swaggerv2.yml
 * Adds v2 end points for Apps to support PUT updates.
 * Rewrites the datastore level tests into a new pattern.
 * V2 routes use entity ID over name as the path parameter.

api/server/trigger_list.go

@@ -0,0 +1,42 @@
+package server
+
+import (
+	"encoding/base64"
+	"net/http"
+
+	"github.com/fnproject/fn/api/models"
+	"github.com/gin-gonic/gin"
+)
+
+func (s *Server) handleTriggerList(c *gin.Context) {
+	ctx := c.Request.Context()
+
+	filter := &models.TriggerFilter{}
+	filter.Cursor, filter.PerPage = pageParams(c, true)
+
+	filter.AppID = c.Query("app_id")
+
+	if filter.AppID == "" {
+		handleErrorResponse(c, models.ErrTriggerMissingAppID)
+	}
+
+	filter.FnID = c.Query("fn_id")
+	filter.Name = c.Query("name")
+
+	triggers, err := s.datastore.GetTriggers(ctx, filter)
+	if err != nil {
+		handleErrorResponse(c, err)
+		return
+	}
+
+	var nextCursor string
+	if len(triggers) > 0 && len(triggers) == filter.PerPage {
+		last := []byte(triggers[len(triggers)-1].ID)
+		nextCursor = base64.RawURLEncoding.EncodeToString(last)
+	}
+
+	c.JSON(http.StatusOK, triggerListResponse{
+		NextCursor: nextCursor,
+		Items:      triggers,
+	})
+}