Abstract exploration aborting logic in `ExploreComponent`

[dandanlen]

It might be nice to have the exploration abort logic integrated with the ExploreComponent base class.

See https://github.com/diffix/explorer/pull/262#pullrequestreview-464107973

The main difficulty might be supporting multiple required components with different types etc, while maintaining enough type information for dependency injection... this might require either some compile-time magic or runtime introspection.

[dandanlen]

Just want to add that if this is ever implemented, it would be good to also add a check of a CancellationToken before running Explore so that cancellation are automatically propagated through all components.

Currently in the ExplorerComponent abstract base class we have:

        public Task<TResult?> ResultAsync => componentTask ??= Task.Run(async () => await Explore());

This could be replaced with (pseudo-code..):

        public Task<TResult?> ResultAsync => componentTask ??= Task.Run(async ()
        {
            // DependentComponents is an IEnumerable<ResultProvider> generated using reflection.
            await Task.WhenAll(DependentComponents.Select(async c => await c.ResultAsync));
            cancellationToken.ThrowIfCancellationRequested();
            await Explore();
        });