runtime-api/ParachainHost: generate_key_ownership_proof

[ordian]

Part of paritytech/substrate#5947.

This will be similar to

polkadot/runtime/kusama/src/lib.rs

Lines 1804 to 1813 in 8801373

	fn generate_key_ownership_proof(
	_set_id: fg_primitives::SetId,
	authority_id: fg_primitives::AuthorityId,
	) -> Option<fg_primitives::OpaqueKeyOwnershipProof> {
	use parity_scale_codec::Encode;
	Historical::prove((fg_primitives::KEY_TYPE, authority_id))
	.map(|p| p.encode())
	.map(fg_primitives::OpaqueKeyOwnershipProof::new)
	}

The AuthorityId in this case could be ValidatorId.

The caller of this API will need to pass the relay_parent of the candidate when CandidateIncluded event was emitted. This way we can guarantee that the session_index of that relay_parent is the one we need for the key ownership proof.

Consideration: note that session_index from the parachains perspective and from sesssion(/historical) can differ. Make sure we're not making an off-by-one error on session boundaries when calculating the key ownership proof.

Alternatively, implement a fix for paritytech/polkadot-sdk#279.