fix: Eliminates string allocations while writing gump packets (#1017)

* Introduces `RawInterpolatedStringHandler` which is exactly the same as `DefaultInterpolatedStringHandler` except it _unsafely exposes_ it's `ReadOnlySpan<char>` buffer. This is useful for writing the string's data without actually building the string.
* Uses this new string interpolation handler in `SpanWriter` to eliminate intermediate strings built. This is immensely useful in eliminating string allocations in writing Gump packets.

Projects/Server/Text/TextEncoding.cs

@@ -105,7 +105,7 @@ namespace Server.Text
         public static int GetBytesUtf8(this ReadOnlySpan<char> str, Span<byte> buffer) => UTF8.GetBytes(str, buffer);
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public static int GetByteLengthForEncoding(Encoding encoding) =>
+        public static int GetByteLengthForEncoding(this Encoding encoding) =>
             encoding.BodyName switch
             {
                 "utf-16BE" => 2,