Add CheckMetadataHash extension

[jsdw]

The new merkalized metadata stuff means that signers will be able to attach a hash of the metadata to the signer payload prior to signing. This ensures that the metadata that they are using to display a TX to the user matches the actual metadata on chain.

The PR for the substrate extension is here: paritytech/polkadot-sdk#4274

The easy solution for now is just to pass None (0u8) to the extension to disable any metadata hash checking.

Since we have the full metadata, we should be able to do better and hash it on first load and pass this hash to the signed extension to be included in any signer payload. We could do this as a step 2, since we'll want to check that we are hashing it properly or whatever.

[bkchr]

Since we have the full metadata, we should be able to do better and hash it on first load and pass this hash to the signed extension to be included in any signer payload. We could do this as a step 2, since we'll want to check that we are hashing it properly or whatever.

You can just use my crate to do the hashing for you.

[jsdw]

Basic work done in #1590 but will leave this open as a reminder to think about adding the full hash to txs via the merkleized-metadata crate