Elastic scaling: relay chain support for new candidate receipts

[sandreim]

The implementation should follow guidelines in RFC #103

The relay chain must accept v2 candidate receipts and use their core information but fallback to the injected core index if the receipt is v1.

Test coverage:

• unit tests for backwards compatibility with non elastic scaling parachains (using old descriptors)
• unit tests backwards compatibility with elastic scaling parachains (MVP)