BTCPay Extensions Part 2 (#2001)

* BTCPay Extensions Part 2

This PR cleans up the extension system a bit in that:
 * It renames the test extension to a more uniform name
 * Allows yo uto have system extensions, which are extensions but bundled by default with the release (and cannot be removed)
 * Adds a tool to help you generate an extension package from a csproj
 * Refactors the UI extension points to a view component
 * Moves some more interfaces to the Abstractions csproj

* Rename to plugins

BTCPayServer.PluginPacker/Program.cs

@@ -0,0 +1,50 @@
+using System;
+using System.IO;
+using System.IO.Compression;
+using System.Linq;
+using System.Reflection;
+using System.Text.Json;
+using BTCPayServer.Contracts;
+
+namespace BTCPayServer.PluginPacker
+{
+    class Program
+    {
+        static void Main(string[] args)
+        {
+            var directory = args[0];
+            var name = args[1];
+            var outputDir = args[2];
+            var outputFile = Path.Combine(outputDir, name);
+            var rootDLLPath = Path.Combine(directory, name +".dll");
+            if (!File.Exists(rootDLLPath) )
+            {
+                throw new Exception($"{rootDLLPath} could not be found");
+            }
+
+            var assembly = Assembly.LoadFrom(rootDLLPath);
+            var extension = GetAllExtensionTypesFromAssembly(assembly).FirstOrDefault();
+            if (extension is null)
+            {
+                throw new Exception($"{rootDLLPath} is not a valid plugin");
+            }
+
+            var loadedPlugin = (IBTCPayServerPlugin)Activator.CreateInstance(extension);
+            var json = JsonSerializer.Serialize(loadedPlugin);
+            Directory.CreateDirectory(outputDir);
+            if (File.Exists(outputFile + ".btcpay"))
+            {
+                File.Delete(outputFile + ".btcpay");
+            }
+            ZipFile.CreateFromDirectory(directory, outputFile + ".btcpay", CompressionLevel.Optimal, false);
+            File.WriteAllText(outputFile + ".btcpay.json", json);
+        }
+        
+        private static Type[] GetAllExtensionTypesFromAssembly(Assembly assembly)
+        {
+            return assembly.GetTypes().Where(type =>
+                typeof(IBTCPayServerPlugin).IsAssignableFrom(type) &&
+                !type.IsAbstract).ToArray();
+        }
+    }
+}