Implement experimental REST API endpoints for plugins (#7279)

* Implement experimental REST API endpoints for plugins

* Updates per feedback and rebase

* Update tests

* Further updates

* Update extraction of plugins

* Use OS temp dir for plugins instead of search path

* Fail extraction on paths that attempt to traverse upward

* Update pluginenv ActivePlugins()

utils/path.go

@@ -0,0 +1,15 @@
+// Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package utils
+
+import (
+	"path/filepath"
+	"strings"
+)
+
+// PathTraversesUpward will return true if the path attempts to traverse upwards by using
+// ".." in the path.
+func PathTraversesUpward(path string) bool {
+	return strings.HasPrefix(filepath.Clean(path), "..")
+}