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/fn_annotator.go

@@ -0,0 +1,63 @@
+package server
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/fnproject/fn/api/models"
+	"github.com/gin-gonic/gin"
+)
+
+//FnAnnotator Is used to inject trigger context (such as request URLs) into outbound trigger resources
+type FnAnnotator interface {
+	// Annotates a trigger on read
+	AnnotateFn(ctx *gin.Context, a *models.App, fn *models.Fn) (*models.Fn, error)
+}
+
+type requestBasedFnAnnotator struct{}
+
+func annotateFnWithBaseURL(baseURL string, app *models.App, fn *models.Fn) (*models.Fn, error) {
+
+	baseURL = strings.TrimSuffix(baseURL, "/")
+	src := strings.TrimPrefix(fn.ID, "/")
+	triggerPath := fmt.Sprintf("%s/invoke/%s", baseURL, src)
+
+	newT := fn.Clone()
+	newAnnotations, err := newT.Annotations.With(models.FnInvokeEndpointAnnotation, triggerPath)
+	if err != nil {
+		return nil, err
+	}
+	newT.Annotations = newAnnotations
+	return newT, nil
+}
+
+func (tp *requestBasedFnAnnotator) AnnotateFn(ctx *gin.Context, app *models.App, t *models.Fn) (*models.Fn, error) {
+
+	//No, I don't feel good about myself either
+	scheme := "http"
+	if ctx.Request.TLS != nil {
+		scheme = "https"
+	}
+
+	return annotateFnWithBaseURL(fmt.Sprintf("%s://%s", scheme, ctx.Request.Host), app, t)
+}
+
+//NewRequestBasedFnAnnotator creates a FnAnnotator that inspects the incoming request host and port, and uses this to generate fn invoke endpoint URLs based on those
+func NewRequestBasedFnAnnotator() FnAnnotator {
+	return &requestBasedFnAnnotator{}
+}
+
+type staticURLFnAnnotator struct {
+	baseURL string
+}
+
+//NewStaticURLFnAnnotator annotates triggers bases on a given, specified URL base - e.g. "https://my.domain" --->  "https://my.domain/t/app/source"
+func NewStaticURLFnAnnotator(baseURL string) FnAnnotator {
+
+	return &staticURLFnAnnotator{baseURL: baseURL}
+}
+
+func (s *staticURLFnAnnotator) AnnotateFn(ctx *gin.Context, app *models.App, trigger *models.Fn) (*models.Fn, error) {
+	return annotateFnWithBaseURL(s.baseURL, app, trigger)
+
+}