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

* Add plugin key value store support

* Add localization strings

* Updates per feedback

model/plugin_key_value_test.go

@@ -0,0 +1,22 @@
+// Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package model
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestPluginKeyIsValid(t *testing.T) {
+	kv := PluginKeyValue{PluginId: "someid", Key: "somekey", Value: []byte("somevalue")}
+	assert.Nil(t, kv.IsValid())
+
+	kv.PluginId = ""
+	assert.NotNil(t, kv.IsValid())
+
+	kv.PluginId = "someid"
+	kv.Key = ""
+	assert.NotNil(t, kv.IsValid())
+}