Dynamic proxy should implement IAsyncDisposable as well

[AArnott]

Our dynamic proxies today implement only IDisposable. By implementing IAsyncDisposable as well, even though local disposal will continue to complete synchronously, clients that know the service's concrete type implements IAsyncDisposable can go on disposing the async way without failing when the interface they have is to an RPC proxy.

vs-streamjsonrpc/src/StreamJsonRpc/ProxyGeneration.cs

Line 88 in bcd8be8

interfaces.Add(typeof(IJsonRpcClientProxy));