Runtime API to create a proof of a leaf of a child storage trie up to the child trie root

[vimukthi-git]

In centrifuge chain we store our data in child storage for a given date. Eg:

set_child_storage(aGivenDate, id, val)

Then someone would ask the chain for a proof of the val up to the root of the child trie. A user of the chain would store this proof to later prove that the val existed on chain at a certain time.

// this function doesn't exist at https://substrate.dev/rustdocs/v1.0/sr_io/index.html
// a user of the chain would store this proof to later prove that the `val` existed on chain at a certain time
proof = child_storage_proof(aGivenDate, id)

Then we would like to delete the items in the storage after a given period, say 1 month, while storing the root of the child trie on chain for proving existence of data at a later time.

// store this root onchain
root = child_storage_root(aGivenDate) 
// delete data
clear_child_storage(aGivenDate)

Would it be possible to expose the child_storage_proof(aGivenDate, id) api in sr_io?

cc @thiolliere @cheme

[rphmeier]

Child-trie state proofs would be generally useful, with broad applications for moving data off-chain especially when combined with APIs for off-chain workers to maintain that data & submit proofs back to chain for sections needed for some computation, although that would be a stretch goal.