Currently a client-context-provider for a client-web-[panel,item,section] must return data synchronously. Support a method for providing asynchronous data instead.

Open questions:

1. How does a client specify async data? (Probably returning a Promise)
2. Do we show loading spinners while the data is loading? Can this be configured?
3. How do we handle errors in these panels?
4. Do we handle timeouts? How?

Related:
https://answers.atlassian.com/questions/15280996/answers/15281927
https://answers.atlassian.com/questions/14812632