IAsyncEnumerable<T> should serialize as an object with token to allow future features

[AArnott]

Instead of serializing is just the token, an IAsyncEnumerable<T> should serialize as an object with a token property so that in the future, the object can carry other significant properties such as an initial array of values (thereby possibly avoiding a round-trip from the receiver just to get the first values that they were perhaps asking for in the first place.

This modifies the spec proposed in #328