Switch node side to v2 candidate receipts (#5679)

on top of https://github.com/paritytech/polkadot-sdk/pull/5423

This PR implements the plumbing work required for
https://github.com/paritytech/polkadot-sdk/issues/5047 . I also added
additional helper methods gated by feature "test" in primitives.

TODO:
- [x] PRDoc

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Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>
Co-authored-by: GitHub Action <action@github.com>