Allow IDisposable or IAsyncDisposable to be implemented by RPC interfaces

[AArnott]

When defining an RPC interface for use in GetProxy<T>, an exception is thrown if T derives from IDisposable because all methods must be asynchronous.

Since the returned proxy actually does implement IDisposable (in terms of disposing the connection), it is important that users of these proxies recognize that the returned object implements IDisposable and dispose of them.

Let's support (and encourage!) RPC interfaces to derive from IDisposable or IAsyncDisposable and document that doing so leads to a local disposal rather than forwarding as an RPC call to the remote side.