Refactor notification system

Introduced `EnvelopeNotificationBase` as a new base class for notifications, providing a unified structure with an abstract `GetTemplateType` method and optional attachments. Added `DocumentReceivedNotification` and updated `DocumentSignedNotification` to inherit from this base class, streamlining attachment handling and template type determination.

Refactored `IEnvelopeNotificationService` to use a generic `Notify` method, allowing for flexibility with different notification types. Updated `FinalizeEnvelopeCommandHandler` to align with the new attachment handling.

Added `EnvelopeNotificationExtensions` for converting attachments to `EmailAttachmentDto` objects and introduced `ReplacePlaceholderExtensions` for dynamic placeholder replacement in email templates using reflection. Removed outdated placeholder logic from `EnvelopeNotificationService`.

EnvelopeGenerator.Infrastructure.Notification/ReplacePlaceholderExtensions.cs

@@ -0,0 +1,47 @@
+using System.Collections.Concurrent;
+using System.Reflection;
+using System.Text.RegularExpressions;
+
+namespace EnvelopeGenerator.Infrastructure.Notification;
+
+public static class ReplacePlaceholderExtensions
+{
+    private sealed record PlaceholderProperty(string Placeholder, PropertyInfo Property);
+
+    private static readonly ConcurrentDictionary<Type, PlaceholderProperty[]> PropertyCache = new();
+
+    public static string ReplacePlaceholders(this string template, object data)
+    {
+        var properties = PropertyCache.GetOrAdd(data.GetType(), CreatePlaceholderProperties);
+
+        foreach (var item in properties)
+        {
+            var value = item.Property.GetValue(data)?.ToString() ?? string.Empty;
+            template = template.Replace(
+                item.Placeholder,
+                value,
+                StringComparison.Ordinal);
+        }
+
+        return template;
+    }
+
+    private static PlaceholderProperty[] CreatePlaceholderProperties(
+        Type type)
+    {
+        return [.. type
+            .GetProperties(BindingFlags.Public | BindingFlags.Instance)
+            .Where(p => p.CanRead && p.GetIndexParameters().Length == 0)
+            .Select(p => new PlaceholderProperty(
+                $"[{ToCapitalSnakeCase(p.Name)}]",
+                p))];
+    }
+
+    private static string ToCapitalSnakeCase(string name)
+    {
+        return Regex.Replace(
+            Regex.Replace(name, @"([A-Z]+)([A-Z][a-z])", "$1_$2"),
+            @"([a-z0-9])([A-Z])",
+            "$1_$2").ToUpperInvariant();
+    }
+}