Determine provider SDK strategy for minimizing API calls

## Data sources

Data sources currently can't take advantage of Etag-based (or really any other) caching. Whenever a data source is refreshed, the SDK does not read any existing state, so etag is ignored, even if it's stored there as a computed attribute.

## Resources

While resources can technically be cached on Etag basis, I think we could do better and make it easier for arbitrary APIs which follow the convention and use the `Etag` header, perhaps through some helper functions that can be plugged in as HTTP transports or something like that.

To start with we could adopt https://github.com/terraform-providers/terraform-provider-github/blob/2eaf170dbd1efa403d3bce08ade826c827a4063b/github/transport.go#L21-L40 in some form to the SDK.