Add CORS support to fn api (#455)

The Gin middleware is being used if one or more Origins are specified. Default setup for each Origin is as follows:

- GET,POST, PUT, HEAD methods allowed
- Credentials share disabled
- Preflight requests cached for 12 hours

Which are the defaults gin-contrib/cors comes with out of the box.

Gin-cors will return a 403 if it gets a request with an Origin header that isn't on its' list. If no Origin header is specified then it will just return the servers response.

Start fn with CORS enabled:

`API_CORS="http://localhost:4000, http://localhost:3000" make run`

vendor/github.com/gin-contrib/cors/config.go

@@ -0,0 +1,83 @@
+package cors
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+)
+
+type cors struct {
+	allowAllOrigins  bool
+	allowCredentials bool
+	allowOriginFunc  func(string) bool
+	allowOrigins     []string
+	exposeHeaders    []string
+	normalHeaders    http.Header
+	preflightHeaders http.Header
+}
+
+func newCors(config Config) *cors {
+	if err := config.Validate(); err != nil {
+		panic(err.Error())
+	}
+	return &cors{
+		allowOriginFunc:  config.AllowOriginFunc,
+		allowAllOrigins:  config.AllowAllOrigins,
+		allowCredentials: config.AllowCredentials,
+		allowOrigins:     normalize(config.AllowOrigins),
+		normalHeaders:    generateNormalHeaders(config),
+		preflightHeaders: generatePreflightHeaders(config),
+	}
+}
+
+func (cors *cors) applyCors(c *gin.Context) {
+	origin := c.Request.Header.Get("Origin")
+	if len(origin) == 0 {
+		// request is not a CORS request
+		return
+	}
+	if !cors.validateOrigin(origin) {
+		c.AbortWithStatus(http.StatusForbidden)
+		return
+	}
+
+	if c.Request.Method == "OPTIONS" {
+		cors.handlePreflight(c)
+		defer c.AbortWithStatus(200)
+	} else {
+		cors.handleNormal(c)
+	}
+
+	if !cors.allowAllOrigins {
+		c.Header("Access-Control-Allow-Origin", origin)
+	}
+}
+
+func (cors *cors) validateOrigin(origin string) bool {
+	if cors.allowAllOrigins {
+		return true
+	}
+	for _, value := range cors.allowOrigins {
+		if value == origin {
+			return true
+		}
+	}
+	if cors.allowOriginFunc != nil {
+		return cors.allowOriginFunc(origin)
+	}
+	return false
+}
+
+func (cors *cors) handlePreflight(c *gin.Context) {
+	header := c.Writer.Header()
+	for key, value := range cors.preflightHeaders {
+		header[key] = value
+	}
+}
+
+func (cors *cors) handleNormal(c *gin.Context) {
+	header := c.Writer.Header()
+	for key, value := range cors.normalHeaders {
+		header[key] = value
+	}
+}