Abstract Store integrations (#2384)

* Decouple Shopify from Store

* Decouple shopify from store blob

* Update BTCPayServer.Tests.csproj

* Make sure shopify obj is set

* make shopify a system plugin

BTCPayServer/Plugins/Shopify/ApiModels/CreateScriptResponse.cs

@@ -0,0 +1,31 @@
+using System;
+using Newtonsoft.Json;
+
+namespace BTCPayServer.Plugins.Shopify.ApiModels
+{
+    public class CreateScriptResponse
+    {
+        [JsonProperty("script_tag")]
+        public ScriptTag ScriptTag { get; set; }
+    }
+    
+    public class ScriptTag    {
+        [JsonProperty("id")]
+        public int Id { get; set; } 
+
+        [JsonProperty("src")]
+        public string Src { get; set; } 
+
+        [JsonProperty("event")]
+        public string Event { get; set; } 
+
+        [JsonProperty("created_at")]
+        public DateTime CreatedAt { get; set; } 
+
+        [JsonProperty("updated_at")]
+        public DateTime UpdatedAt { get; set; } 
+
+        [JsonProperty("display_scope")]
+        public string DisplayScope { get; set; } 
+    }
+}