Consider moving `GetResponseAsync<T>` down to M.E.AI.Abstractions

[stephentoub]

We've heard of a few cases recently where folks have avoided using GetResponseAsync<T> because it would mean adding a dependency on M.E.AI rather than M.E.AI.Abstractions, or at least they thought twice about using it for that reason. As GetResponseAsync<T> shouldn't actually bring in any additional dependencies and as it doesn't carry a large footprint, we should consider pushing it down to M.E.AI.Abstractions.