[MM-53968] Includes mattermost-plugin-api into the mono repo (#24235)

Include https://github.com/mattermost/mattermost-plugin-api into the mono repo

Co-authored-by: Jesse Hallam <jesse.hallam@gmail.com>
Co-authored-by: Michael Kochell <mjkochell@gmail.com>
Co-authored-by: Alejandro García Montoro <alejandro.garciamontoro@gmail.com>
Co-authored-by: Ben Schumacher <ben.schumacher@mattermost.com>
Co-authored-by: Alex Dovenmuehle <alex.dovenmuehle@mattermost.com>
Co-authored-by: Michael Kochell <6913320+mickmister@users.noreply.github.com>
Co-authored-by: Christopher Poile <cpoile@gmail.com>
Co-authored-by: İlker Göktuğ Öztürk <ilkergoktugozturk@gmail.com>
Co-authored-by: Shota Gvinepadze <wineson@gmail.com>
Co-authored-by: Ali Farooq <ali.farooq0@pm.me>
Co-authored-by: Maria A Nunez <maria.nunez@mattermost.com>
Co-authored-by: Daniel Espino García <larkox@gmail.com>
Co-authored-by: Christopher Speller <crspeller@gmail.com>
Co-authored-by: Alex Dovenmuehle <adovenmuehle@gmail.com>
Co-authored-by: Szymon Gibała <szymongib@gmail.com>
Co-authored-by: Lev <1187448+levb@users.noreply.github.com>
Co-authored-by: Jason Frerich <jason.frerich@mattermost.com>
Co-authored-by: Agniva De Sarker <agnivade@yahoo.co.in>
Co-authored-by: Artur M. Wolff <artur.m.wolff@gmail.com>
Co-authored-by: Madhav Hugar <16546715+madhavhugar@users.noreply.github.com>
Co-authored-by: Joe <security.joe@pm.me>
Co-authored-by: Ibrahim Serdar Acikgoz <serdaracikgoz86@gmail.com>
Co-authored-by: José Peso <trilopin@users.noreply.github.com>

server/public/pluginapi/experimental/oauther/oauth2_complete.go

@@ -0,0 +1,105 @@
+// Copyright (c) 2019-present Mattermost, Inc. All Rights Reserved.
+// See License for license information.
+
+package oauther
+
+import (
+	"context"
+	"fmt"
+	"net/http"
+	"strings"
+)
+
+func (o *oAuther) oauth2Complete(w http.ResponseWriter, r *http.Request) {
+	if r.Method != http.MethodGet {
+		http.Error(w, "Method not allowed", http.StatusMethodNotAllowed)
+		return
+	}
+	authedUserID := r.Header.Get("Mattermost-User-ID")
+	if authedUserID == "" {
+		o.logger.Debugf("oauth2Complete: reached by non authed user")
+		http.Error(w, "Not authorized", http.StatusUnauthorized)
+		return
+	}
+	code := r.URL.Query().Get("code")
+	if code == "" {
+		o.logger.Debugf("oauth2Complete: reached with no code")
+		http.Error(w, "Bad request", http.StatusBadRequest)
+		return
+	}
+	state := r.URL.Query().Get("state")
+
+	var storedState string
+	err := o.store.Get(o.getStateKey(authedUserID), &storedState)
+	if err != nil {
+		o.logger.Warnf("oauth2Complete: cannot get state, err=%s", err.Error())
+		http.Error(w, "Internal server error", http.StatusInternalServerError)
+		return
+	}
+
+	if storedState != state {
+		o.logger.Debugf("oauth2Complete: state mismatch")
+		o.logger.Debugf("received state '%s'; expected state '%s%", state, storedState)
+		http.Error(w, "Not authorized", http.StatusUnauthorized)
+		return
+	}
+
+	userID := strings.Split(state, "_")[1]
+	if userID != authedUserID {
+		o.logger.Debugf("oauth2Complete: authed user mismatch")
+		http.Error(w, "Not authorized", http.StatusUnauthorized)
+		return
+	}
+
+	ctx := context.Background()
+	token, err := o.config.Exchange(ctx, code)
+	if err != nil {
+		o.logger.Warnf("oauth2Complete: could not generate token, err=%s", err.Error())
+		http.Error(w, "Not authorized", http.StatusUnauthorized)
+		return
+	}
+
+	var payload []byte
+	err = o.store.Get(o.getPayloadKey(userID), &payload)
+	if err != nil {
+		o.logger.Errorf("oauth2Complete: could not fetch payload, err=&s", err.Error())
+		http.Error(w, "Internal server error", http.StatusInternalServerError)
+		return
+	}
+
+	ok, err := o.store.Set(o.getTokenKey(userID), token)
+	if err != nil {
+		o.logger.Errorf("oauth2Complete: cannot store the token, err=%s", err.Error())
+		http.Error(w, "Internal server error", http.StatusInternalServerError)
+		return
+	}
+	if !ok {
+		o.logger.Errorf("oauth2Complete: cannot store token without error")
+		http.Error(w, "Internal server error", http.StatusInternalServerError)
+		return
+	}
+
+	html := fmt.Sprintf(`
+		<!DOCTYPE html>
+		<html>
+			<head>
+				<script>
+					window.close();
+				</script>
+			</head>
+			<body>
+				<p>%s</p>
+			</body>
+		</html>
+		`, o.connectedString)
+
+	w.Header().Set("Content-Type", "text/html")
+	_, err = w.Write([]byte(html))
+	if err != nil {
+		o.logger.Errorf("oauth2Complete: error writing response, err=%s", err.Error())
+	}
+
+	if o.onConnect != nil {
+		o.onConnect(userID, *token, payload)
+	}
+}