fix: Reverts GetAccount to use IAccount. Adds IAccount to serialization. (#1565)

### Summary

- Fixes usernames not being `Intern`ed
- Reverts methods related to getting accounts from returning `Account` to `IAccount`.
- Makes `IAccount` also `ISerializable`
- Adds `IGenericReader.ReadAccount()` and `IGenericWriter.Write(IAccount)` -> The read method supports the original serialization of username, and using `IAccount.Serial`. The write method only serializes the `Serial`.
- Exposes `ReadStringRaw()` to allow some advanced scenarios.

Projects/UOContent/Misc/SerializationExt.cs

@@ -0,0 +1,29 @@
+using Server.Accounting;
+
+namespace Server.Misc;
+
+public static class SerializationExt
+{
+    public static IAccount ReadAccount(this IGenericReader reader)
+    {
+        return reader.ReadByte() switch
+        {
+            0 => null,
+            1 => Accounts.GetAccount(reader.ReadStringRaw()),
+            2 => Accounts.FindAccount(reader.ReadSerial())
+        };
+    }
+
+    public static void Write(this IGenericWriter writer, IAccount acct)
+    {
+        if (acct == null)
+        {
+            writer.Write((byte)0);
+        }
+        else
+        {
+            writer.Write((byte)2);
+            writer.Write(acct.Serial);
+        }
+    }
+}