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_test.go

@@ -0,0 +1,31 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package utils
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestPathTraversesUpward(t *testing.T) {
+	cases := []struct {
+		input    string
+		expected bool
+	}{
+		{"../test/path", true},
+		{"../../test/path", true},
+		{"../../test/../path", true},
+		{"test/../../path", true},
+		{"test/path/../../", false},
+		{"test", false},
+		{"test/path", false},
+		{"test/path/", false},
+		{"test/path/file.ext", false},
+	}
+
+	for _, c := range cases {
+		assert.Equal(t, c.expected, PathTraversesUpward(c.input), c.input)
+	}
+}