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.Test.csproj

@@ -21,6 +21,7 @@
     <DefineConstants>TRACE;DEBUG</DefineConstants>
     <ErrorReport>prompt</ErrorReport>
     <WarningLevel>4</WarningLevel>
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
   </PropertyGroup>
   <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|AnyCPU' ">
     <DebugType>pdbonly</DebugType>
@@ -29,6 +30,7 @@
     <DefineConstants>TRACE</DefineConstants>
     <ErrorReport>prompt</ErrorReport>
     <WarningLevel>4</WarningLevel>
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
   </PropertyGroup>
   <PropertyGroup>
     <StartupObject />