Add async support for DotNet pickers

[DanDiplo]

I use the DotNet pickers a lot and they are fantastic for pulling in data from external data sources. But more and more these sources use async requests - for instance, anything using HttpClient will be async. But the IDotNetDataSource interface only has a synchronous method for getting editor data items. It would be really cool if there was also an option for an async version of this ie.

public async Task<IEnumerable<KeyValuePair<string, string>>> GetEditorDataItems(int contextId)

Not sure how possible this would be, but if it is that would be great. Thanks for listening.