Add overloads for C# ByteBuffer/FlatBufferBuilder to allow adding vector blocks from ArraySegments or IntPtr (#7193)

* Add overloads to Add/Put for ArraySegment and IntPtr

In order to allow using code to reduce memory allocations, add overloads to ByteBuffer's and FlatBuffersBuilder's Put/Add methods that take ArraySegment<T> or IntPtr respectively.
Also, adaptions to the c# code generator in flatc to emit corresponding CreateVectorBlock() overloads

* Add missing files generated with generate_code.py

The previous commit changed the C# code generate, but didn't contain the updated generated test files.

* Incorporate review findings

(1) Adhere to 80 characters limit.
(2) In FlatBufferBuilder.Add(IntPtr,int), move zero length check topmost and add sanity check against negative input

tests/FlatBuffers.Test/FlatBuffers.Core.Test.csproj

@@ -19,6 +19,14 @@
     <DefineConstants>$(DefineConstants);ENABLE_SPAN_T</DefineConstants>
   </PropertyGroup>
 
+  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|AnyCPU'">
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+  </PropertyGroup>
+
+  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Release|AnyCPU'">
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+  </PropertyGroup>
+
   <ItemGroup>
     <Compile Include="..\..\net\FlatBuffers\ByteBuffer.cs">
       <Link>FlatBuffers\ByteBuffer.cs</Link>