bot: add birthday command

PluralKit.Bot/Utils.cs

@@ -9,6 +9,7 @@ using Discord.Commands;
 using Discord.Commands.Builders;
 using Discord.WebSocket;
 using Microsoft.Extensions.DependencyInjection;
+using NodaTime;
 
 namespace PluralKit.Bot
 {
@@ -32,6 +33,19 @@ namespace PluralKit.Bot
         }
     }
 
+    class PassthroughTypeHandler<T> : SqlMapper.TypeHandler<T>
+    {
+        public override void SetValue(IDbDataParameter parameter, T value)
+        {
+            parameter.Value = value;
+        }
+
+        public override T Parse(object value)
+        {
+            return (T) value;
+        }
+    }
+
     class PKSystemTypeReader : TypeReader
     {
         public override async Task<TypeReaderResult> ReadAsync(ICommandContext context, string input, IServiceProvider services)