Runtime API: make backing_state return multiple candidates pending availability

[alindima]

This is being used by prospective-parachains when building fragment trees. Make the runtime API return an ordered collection of candidates pending availability for the given paraid.

[alindima]

the interface already supports this, just need to alter the implementation

[alindima]

The assumption that this returns an ordered collection of candidates is being made, for example here:

polkadot-sdk/polkadot/node/core/prospective-parachains/src/lib.rs

Line 354 in 53f615d

async fn preprocess_candidates_pending_availability<Context>(

[alindima]

on a second thought, this can't be done without #3131