PLT-8131 (part2) Add plugin key value store support (#7902)

* Add plugin key value store support

* Add localization strings

* Updates per feedback

store/sqlstore/plugin_store_test.go

@@ -0,0 +1,14 @@
+// Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package sqlstore
+
+import (
+	"testing"
+
+	"github.com/mattermost/mattermost-server/store/storetest"
+)
+
+func TestPluginStore(t *testing.T) {
+	StoreTest(t, storetest.TestPluginStore)
+}