improve mockability of generated code

[jeffMSFT]

The code generator emits a lot of boilerplate that could be captured in a base interface, and thus simplify the task of mocking a collection proxy. For example,

    public interface IItemCollection : ProxyExtensions.IReadOnlyQueryableSetBase<Models.IItem>

    {

        Models.IItemFetcher GetById(System.Guid id);

        Task<ProxyExtensions.IPagedCollection<Models.IItem>> ExecuteAsync();

        Task AddItemAsync(Models.IItem item, System.Boolean deferSaveChanges = false);

        Models.IItemFetcher this[System.Guid id]    { get; }
    }

...could become

    // unique to my model
    public partial interface IItemCollection : IEntityCollection<Models.IItem>
    {
        Models.IEntityFetcher<Models.IItem> GetById(System.Guid id);
    }

    // shared by all entity definitions, perhaps in ProxyExtensions assembly
    public interface IEntityCollection<T> : ProxyExtensions.IReadOnlyQueryableSetBase<T>
    {
        Task<ProxyExtensions.IPagedCollection<T>> ExecuteAsync();

        Task AddItemAsync(T item, System.Boolean deferSaveChanges = false);

        Models.IEntityFetcher<T> this[System.Guid id]    { get; }
    }

    public interface IEntityFetcher<TSource>

    {
        Task<T> ExecuteAsync();

        T Expand<TTarget>(System.Linq.Expressions.Expression<System.Func<TSource, TTarget>> navigationPropertyAccessor);
    }

Is the team amenable to this kind of refactoring?

[MIchaelMainer]

@jeffMSFT Thank you for the issue. Yes, we may be open to this at some point in the future. We just aren't in a position right now to take pull requests and review them.