fix(houses): Converts house packets (1 of 2) (#502)

- [X] Adds `ToSpan()` for SpanWriter.
- [X] Moves house files to UOContent/Multis/Houses
- [X] Adds house packets

### SpanWriter.ToSpan()
`ToSpan()` returns an `SpanOwner` struct which allows access to the buffer as a Span by calling `Span`.

Example:
```cs
public SpanOwner CreatePacket()
{
  var writer = new SpanWriter(0x400);
  // write some stuff
  return writer.ToSpan();
}
```
In this example, we are creating a packet and returning the `SpanOwner` for caching. This means in the future we can do something like this:
```cs
ns.Send(_cachedPacket.Span);
```
Where `_cachedPacket` is the `SpanOwner`

Notes:
Do not use a SpanWriter, or attempt to dispose of it after calling `ToSpan()`. Doing so will probably cause an NPE.

Projects/UOContent.Tests/Tests/Multis/Houses/HousePacketTests.cs

@@ -0,0 +1,52 @@
+using System;
+using Server.Multis;
+using Server.Network;
+using Server.Tests;
+using Server.Tests.Network;
+using Xunit;
+
+namespace UOContent.Tests
+{
+    public class HousePacketTests
+    {
+        [Theory]
+        [InlineData(0x1001u)]
+        public void TestBeginHouseCustomization(uint serial)
+        {
+            var expected = new BeginHouseCustomization(serial).Compile();
+
+            var ns = PacketTestUtilities.CreateTestNetState();
+            ns.SendBeginHouseCustomization(serial);
+
+            var result = ns.SendPipe.Reader.TryRead();
+            AssertThat.Equal(result.Buffer[0].AsSpan(0), expected);
+        }
+
+        [Theory]
+        [InlineData(0x1001u)]
+        public void TestEndHouseCustomization(uint serial)
+        {
+            var expected = new EndHouseCustomization(serial).Compile();
+
+            var ns = PacketTestUtilities.CreateTestNetState();
+            ns.SendEndHouseCustomization(serial);
+
+            var result = ns.SendPipe.Reader.TryRead();
+            AssertThat.Equal(result.Buffer[0].AsSpan(0), expected);
+        }
+
+        [Theory]
+        [InlineData(0x1001u, 0)]
+        [InlineData(0x1001u, 100)]
+        public void TestDesignStateGeneral(uint serial, int revision)
+        {
+            var expected = new DesignStateGeneral(serial, revision).Compile();
+
+            var ns = PacketTestUtilities.CreateTestNetState();
+            ns.SendDesignStateGeneral(serial, revision);
+
+            var result = ns.SendPipe.Reader.TryRead();
+            AssertThat.Equal(result.Buffer[0].AsSpan(0), expected);
+        }
+    }
+}

Projects/UOContent.Tests/Tests/Multis/Houses/HousePackets.cs

@@ -0,0 +1,46 @@
+namespace Server.Network
+{
+    public class BeginHouseCustomization : Packet
+    {
+        public BeginHouseCustomization(Serial house) : base(0xBF)
+        {
+            EnsureCapacity(17);
+
+            Stream.Write((short)0x20);
+            Stream.Write(house);
+            Stream.Write((byte)0x04);
+            Stream.Write((ushort)0x0000);
+            Stream.Write((ushort)0xFFFF);
+            Stream.Write((ushort)0xFFFF);
+            Stream.Write((byte)0xFF);
+        }
+    }
+
+    public class EndHouseCustomization : Packet
+    {
+        public EndHouseCustomization(Serial house) : base(0xBF)
+        {
+            EnsureCapacity(17);
+
+            Stream.Write((short)0x20);
+            Stream.Write(house);
+            Stream.Write((byte)0x05);
+            Stream.Write((ushort)0x0000);
+            Stream.Write((ushort)0xFFFF);
+            Stream.Write((ushort)0xFFFF);
+            Stream.Write((byte)0xFF);
+        }
+    }
+
+    public sealed class DesignStateGeneral : Packet
+    {
+        public DesignStateGeneral(Serial house, int revision) : base(0xBF)
+        {
+            EnsureCapacity(13);
+
+            Stream.Write((short)0x1D);
+            Stream.Write(house);
+            Stream.Write(revision);
+        }
+    }
+}