Implement Pooling of MemoryStream in DecompressDecodeMessage and DecompressDecodeQueryUpdate

Originally included as a TODO comment in https://github.com/clockworklabs/SpacetimeDB/commit/5ad9baa3049ff575e87668c61f65c139352930d7, when either DecompressDecodeMessage or DecompressDecodeQueryUpdate are called in the C# SDK, the following is ran:

// DO NOT TRY TO TAKE THIS OUT. The BrotliStream ReadByte() implementation allocates an array
// PER BYTE READ. You have to do it all at once to avoid that problem.
MemoryStream memoryStream = new MemoryStream();
decompressedStream.CopyTo(memoryStream);
memoryStream.Seek(0, SeekOrigin.Begin);
return new ServerMessage.BSATN().Read(new BinaryReader(memoryStream));

This MemoryStream may be eligible for pooling, in order to avoid a new allocation every time this runs.

Locate the C# SDK implementations of DecompressDecodeMessage and DecompressDecodeQueryUpdate and read how each creates and consumes the MemoryStream. Check the surrounding decompression and decoding code first, then identify the SDK's existing pooling facilities. Done means both paths avoid a fresh MemoryStream allocation while preserving the current decoding behavior.