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

* Add plugin key value store support

* Add localization strings

* Updates per feedback

app/plugin_test.go

@@ -0,0 +1,35 @@
+// Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package app
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestPluginKeyValueStore(t *testing.T) {
+	th := Setup().InitBasic()
+	defer th.TearDown()
+
+	pluginId := "testpluginid"
+
+	assert.Nil(t, th.App.SetPluginKey(pluginId, "key", []byte("test")))
+	ret, err := th.App.GetPluginKey(pluginId, "key")
+	assert.Nil(t, err)
+	assert.Equal(t, []byte("test"), ret)
+
+	// Test inserting over existing entries
+	assert.Nil(t, th.App.SetPluginKey(pluginId, "key", []byte("test2")))
+
+	// Test getting non-existent key
+	ret, err = th.App.GetPluginKey(pluginId, "notakey")
+	assert.Nil(t, err)
+	assert.Nil(t, ret)
+
+	assert.Nil(t, th.App.DeletePluginKey(pluginId, "stringkey"))
+	assert.Nil(t, th.App.DeletePluginKey(pluginId, "intkey"))
+	assert.Nil(t, th.App.DeletePluginKey(pluginId, "postkey"))
+	assert.Nil(t, th.App.DeletePluginKey(pluginId, "notrealkey"))
+}