Investigate: Support Merkleized Metadata

[Niederb]

RFC-0078 Merkelized Metadata proposes to include a metadata hash in the extrinsic signature to improve trust in the protocol:
https://polkadot-fellows.github.io/RFCs/approved/0078-merkleized-metadata.html

• PR to provide this feature: Add CheckMetadataHash extension polkadot-fellows/runtimes#337
• Subxt is discussing whether to support this: Support merkalised metadata? paritytech/subxt#1586
• The feature was activated on Kusama: https://kusama.subsquare.io/referenda/406

Open questions

• Why did we not detect this in our builds?
  • The docker image we use for testing (paritypr/substrate:latest) is 10 months old. This seems a bit outdated
    • https://hub.docker.com/layers/paritypr/substrate/latest/images/sha256-a4e5fc063bee26dce1f8feb998712111be71b130118148fad1efc75d371bf97c?context=explore
    • Also see Docker image paritypr/substrate:latest not updated anymore? paritytech/polkadot-sdk#4789
    • After some digging I found out that this is indeed the reason why we did not notice this breaking change. Testing with other docker images I get the following results
      • paritypr/substrate:master-ae864e6a broken
      • paritypr/substrate:8dbe4ee8 broken
      • paritypr/substrate:master-eda98954 works
      • paritypr/substrate:master-ad54bc36 works
      • Commit 8dbe4ee8 introduced the CheckMetadataHash extension
        • paritytech/polkadot-sdk@8dbe4ee
  • Could this be improved in the future?

[masapr]

I created a new issue #778 for the up-to-date images

[haerdib]

Link to Code of Parity: https://github.com/paritytech/polkadot-sdk/blob/master/substrate/frame/metadata-hash-extension/src/lib.rs#L105