[c#] Add OutputBuffer virtual resize buffer method

`OutputBuffer` always uses `new byte[]` to create/resize its internal
buffer. Implementing a custom `IOutputBuffer` that uses a different
allocation scheme requires duplicating a lot of code in `OutputBuffer` and
`IntegerHelper`.

Add a virtual method `OutputBuffer.ResizeBuffer` that can be overridden to
use buffer allocators other than `new byte[]` (e.g.
`ArrayPool<byte>.Rent()`)

CHANGELOG.md

@@ -15,7 +15,7 @@ different versioning scheme, following the Haskell community's
 
 * IDL core version: TBD
 * C++ version: TBD (minor version bump needed)
-* C# NuGet version: TBD
+* C# NuGet version: (minor version bump needed)
 * `gbc` & compiler library: TBD
 
 ### C++ ###
@@ -34,6 +34,13 @@ different versioning scheme, following the Haskell community's
 * Added an ability to apply transform to a schema.
 * Added `noexcept` to `bond::blob`'s non-throwing functions.
 
+### C# ###
+
+* Added virtual method `OutputBuffer.ResizeBuffer` that can be overridden to
+  use buffer allocators other than `new byte[]` (e.g.
+  `ArrayPool<byte>.Rent()`). ([Pull request
+  \#1128](https://github.com/microsoft/bond/pull/1128))
+
 ## 9.0.5: 2021-04-14 ##
 
 * IDL core version: 3.0

cs/src/core/io/safe/OutputBuffer.cs

@@ -36,14 +36,22 @@ public virtual long Position
         }
 
         public OutputBuffer(int length = 64 * 1024)
-            : this(new byte[length])
-        {}
+            : this()
+        {
+            buffer = ResizeBuffer(null, length);
+            this.length = buffer.Length;
+        }
 
         public OutputBuffer(byte[] buffer)
+            : this()
         {
-            Debug.Assert(BitConverter.IsLittleEndian);
             this.buffer = buffer;
             length = buffer.Length;
+        }
+
+        private OutputBuffer()
+        {
+            Debug.Assert(BitConverter.IsLittleEndian);
             position = 0;
         }
 
@@ -233,7 +241,27 @@ internal virtual void Grow(int count)
                 length = minLength;
             }
 
-            Array.Resize(ref buffer, length);
+            buffer = ResizeBuffer(buffer, length);
+            length = buffer.Length;
+        }
+
+        /// <summary>
+        /// Resize the internal buffer.
+        /// </summary>
+        /// <param name="buffer">Existing buffer.</param>
+        /// <param name="newSize">New buffer size.</param>
+        /// <returns>The new buffer.</returns>
+        /// <remarks>
+        /// <para>
+        /// Implementations are responsible for ensuring that the new buffer
+        /// is at least <paramref name="newSize"/> bytes large and that the
+        /// bytes in <paramref name="buffer"/> are copied to the new buffer.
+        /// </para>
+        /// </remarks>
+        protected virtual byte[] ResizeBuffer(byte[] buffer, int newSize)
+        {
+            Array.Resize(ref buffer, newSize);
+            return buffer;
         }
 
         #region layouts