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.go

@@ -0,0 +1,26 @@
+// Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package model
+
+import (
+	"net/http"
+)
+
+type PluginKeyValue struct {
+	PluginId string `json:"plugin_id"`
+	Key      string `json:"key" db:"PKey"`
+	Value    []byte `json:"value" db:"PValue"`
+}
+
+func (kv *PluginKeyValue) IsValid() *AppError {
+	if len(kv.PluginId) == 0 {
+		return NewAppError("PluginKeyValue.IsValid", "model.plugin_key_value.is_valid.plugin_id.app_error", nil, "key="+kv.Key, http.StatusBadRequest)
+	}
+
+	if len(kv.Key) == 0 {
+		return NewAppError("PluginKeyValue.IsValid", "model.plugin_key_value.is_valid.key.app_error", nil, "key="+kv.Key, http.StatusBadRequest)
+	}
+
+	return nil
+}