webhooks: implemented Uber webhooks

[odeke-em]

Fixes #29.

Implemented a package for Uber webhook reception and verification.
The example features a fully runnable HTTPS server with Let's Encrypt
certs, just change the domains and the rest is simple. Make sure
to set in your environment

• Client ID: UBER_APP_OAUTH2_CLIENT_ID
• Client Secret: UBER_APP_OAUTH2_CLIENT_SECRET

Sample runnable example pasted here for easy lookup

package main

import (
  "flag"
  "fmt"
  "log"
  "net/http"

  "golang.org/x/crypto/acme/autocert"

  "github.com/orijtech/otils"
  "github.com/orijtech/uber/uberhook"
)

func main() {
  var http1 bool
  var http1Port int
  flag.BoolVar(&http1, "http1", false, "run the server as an HTTP1
server: useful for local dev")
  flag.IntVar(&http1Port, "port", 8888, "the port to run the HTTP1
server on")
  flag.Parse()

  webhook, err := uberhook.New()
  if err != nil {
    log.Fatal(err)
  }

  mux := http.NewServeMux()
  mux.Handle("/", webhook.Middleware(http.HandlerFunc(func(w
http.ResponseWriter, r *http.Request) {
    defer r.Body.Close()
    event, err := uberhook.FparseEvent(r.Body)

    if err != nil {
      http.Error(w, err.Error(), http.StatusBadRequest)
      return
    }
    log.Printf("Got an event from Uber: %#v\n", event)
    fmt.Fprintf(w, "Successfully retrieved event!\n")
  })))

  if http1 {
    addr := fmt.Sprintf(":%d", http1Port)
    log.Printf("Running as an http1 server on: %s\n", addr)
    if err := http.ListenAndServe(addr, mux); err != nil {
      log.Fatal(err)
    }
    return
  }

  log.Printf("Running as an http2 server!")
  go func() {
    nonHTTPSHandler :=
otils.RedirectAllTrafficTo("https://uberhook.orijtech.com")
    if err := http.ListenAndServe(":80", nonHTTPSHandler); err != nil {
      log.Fatal(err)
    }
  }()

  domains := []string{
    "uberhook.orijtech.com",
    "www.uberhook.orijtech.com",
  }

  log.Fatal(http.Serve(autocert.NewListener(domains...), mux))
}