fix: add oauth2 examples [issue ]

_examples/oauth2/README.md

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+# Oauth2 and OpenID 
+Oauth2 and OpenID is an example usage of goguardian oauth2 package. the example provides a simple REST API for a login using google oauth2 and an api to query a book author by id.
+
+# Getting Started
+1. [https://www.balbooa.com/gridbox-documentation/how-to-get-google-client-id-and-client-secret](create google oauth2 client id and secret)
+2. run example 
+```sh
+CLIENT_ID="" CLIENT_SECRET="" go run main.go 
+```
+3. open web browser and hit http://127.0.0.1:8080/v1/auth/login.
+4. login using your google.
+5. copy the token
+6. http://127.0.0.1:8080/v1/book/1449311601?token=<paste token>
+
+**Note:** you may encounter `strategies/oauth2/jwt: claims: standard claims issued at a future time` so wait a few seconds until token activated.

_examples/oauth2/main.go

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+// Copyright 2022 The Go-Guardian. All rights reserved.
+// Use of this source code is governed by a MIT
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"context"
+	"crypto/rand"
+	"encoding/base64"
+	"fmt"
+	"log"
+	"net/http"
+	"os"
+	"time"
+
+	"github.com/gorilla/mux"
+	"github.com/shaj13/libcache"
+	_ "github.com/shaj13/libcache/fifo"
+	"golang.org/x/oauth2"
+
+	"github.com/shaj13/go-guardian/v2/auth"
+	"github.com/shaj13/go-guardian/v2/auth/strategies/oauth2/jwt"
+	"github.com/shaj13/go-guardian/v2/auth/strategies/token"
+)
+
+var strategy auth.Strategy
+var oauthConfig = &oauth2.Config{
+	RedirectURL:  "http://127.0.0.1:8080/v1/auth/grant",
+	ClientID:     os.Getenv("CLIENT_ID"),
+	ClientSecret: os.Getenv("CLIENT_SECRET"),
+	// OpenID Connect (OIDC) scopes are used by an application during authentication
+	// To authorize access to a user's details, like name and picture.
+	// Each scope returns a set of user attributes, which are called claims.
+	// The scopes an application should request depend on which user attributes the application needs.
+	// Once the user authorizes the requested scopes, the claims are returned in an ID Token.
+	Scopes: []string{"openid"},
+	Endpoint: oauth2.Endpoint{
+		AuthURL:   "https://accounts.google.com/o/oauth2/auth",
+		TokenURL:  "https://oauth2.googleapis.com/token",
+		AuthStyle: 0,
+	},
+}
+
+func main() {
+	setupGoGuardian()
+	router := mux.NewRouter()
+	router.HandleFunc("/v1/book/{id}", middleware(http.HandlerFunc(getBookAuthor))).Methods("GET")
+	router.HandleFunc("/v1/auth/login", http.HandlerFunc(oauth2Login)).Methods("GET")
+	router.HandleFunc("/v1/auth/grant", http.HandlerFunc(oauth2Grant)).Methods("GET")
+	log.Println("server started and listening on http://127.0.0.1:8080")
+	http.ListenAndServe("127.0.0.1:8080", router)
+}
+
+func oauth2Login(w http.ResponseWriter, r *http.Request) {
+	expiration := time.Now().Add(20 * time.Minute)
+	buf := make([]byte, 16)
+	rand.Read(buf)
+	state := base64.URLEncoding.EncodeToString(buf)
+	cookie := http.Cookie{Name: "oauthstate", Value: state, Expires: expiration}
+	http.SetCookie(w, &cookie)
+	url := oauthConfig.AuthCodeURL(state)
+	http.Redirect(w, r, url, http.StatusTemporaryRedirect)
+}
+
+func oauth2Grant(w http.ResponseWriter, r *http.Request) {
+	oauthState, _ := r.Cookie("oauthstate")
+	code := r.FormValue("code")
+	if r.FormValue("state") != oauthState.Value {
+		log.Println("invalid oauth google state")
+		http.Redirect(w, r, "/", http.StatusTemporaryRedirect)
+		return
+	}
+
+	// use code to get token.
+	token, err := oauthConfig.Exchange(context.Background(), code)
+	if err != nil {
+		http.Error(w, err.Error(), http.StatusInternalServerError)
+		return
+	}
+
+	w.Write([]byte("successful logon; \n"))
+	w.Write([]byte("token: \n"))
+	w.Write([]byte(token.Extra("id_token").(string)))
+	w.Write([]byte("\n Note: use the granted token in url query (?token=);"))
+}
+
+func getBookAuthor(w http.ResponseWriter, r *http.Request) {
+	vars := mux.Vars(r)
+	id := vars["id"]
+	books := map[string]string{
+		"1449311601": "Ryan Boyd",
+		"148425094X": "Yvonne Wilson",
+		"1484220498": "Prabath Siriwarden",
+	}
+	body := fmt.Sprintf("Author: %s \n", books[id])
+	w.Write([]byte(body))
+}
+
+func setupGoGuardian() {
+	cache := libcache.FIFO.New(0)
+	cache.SetTTL(time.Minute * 5)
+	cache.RegisterOnExpired(func(key, _ interface{}) {
+		cache.Peek(key)
+	})
+	parser := token.QueryParser("token")
+	opt := token.SetParser(parser)
+	// Use jwt.SetClaimResolver()
+	// To extract additionals user's details.
+	strategy = jwt.New("https://www.googleapis.com/oauth2/v3/certs", cache, opt)
+}
+
+func middleware(next http.Handler) http.HandlerFunc {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		log.Println("Executing Auth Middleware")
+		user, err := strategy.Authenticate(r.Context(), r)
+		if err != nil {
+			fmt.Println(err)
+			code := http.StatusUnauthorized
+			http.Error(w, http.StatusText(code), code)
+			return
+		}
+		log.Printf("User %s Authenticated\n", user.GetUserName())
+		r = auth.RequestWithUser(user, r)
+		next.ServeHTTP(w, r)
+	})
+}