Distinguish expandability for a collection vs expandability for an item within a collection

Gareth Jones opened https://github.com/oasis-tcs/odata/issues/12:

It's very useful to be able to distinguish capabilities of a single entity in a collection vs the capabilities of a collection.

This is embodied in ReadRestrictions vs ReadByKeyRestrictions for support for GET foos vs GET foos/{fooId}

In practice we've found that implementing $expand on a single entity is frequently cost-effective, whereas implementing it across a whole collection is not. Consequently we'd like to be able to qualify ExpandRestrictions with something akin to an ExpandByKeyRestrictions.

A cheap and cheerful variant of this would be a boolean control flag in ExpandRestrictions, but it misses the possibility that the expandable set of nav properties varies between single/collection.

For clarity, we need a vocabulary annotation that allows us to express

1. NOT Supported: GET /foos?$expand=bars
2. Supported: GET /foos/{fooId}?$expand=bars

Another issue of ExpandRestrictions is ODATA-1502.