Add option to fetcher hooks to defer loading of resources

[Frewacom]

There is currently no good way of fetching data conditionally. As soon as you use one of the fetcher hooks, the data will be instantly fetched (either via request or from cache). Sometimes you want to only load data on user input. The only way of doing this now is to move the fetcher hook to a separate component and then conditionally render it. This is quite ugly and it would be much better if it would be supported natively by the SDK.