Add generate and verify logic for `AncestryProof` (#4430)

Extracting the logic for generating and verifying ancestry proofs from
this PR: https://github.com/paritytech/polkadot-sdk/pull/1903 with small
adjustments

@Lederstrumpf I added you as a co-author to each commit. Please let me
know if you want me to also associate an e-mail address with your name