Support `CheckMetadata` signed extension

[clangenb]

I have introduced a hacky way for it to hotfix our setup here: https://github.com/encointer/polkadart/tree/cl/add-checkmetadata-extension

Maybe I have some time soon to properly implement this and upstream, but I am not sure when.

[leonardocustodio]

Hello @clangenb , thank you for the heads up. May I ask is that a new Polkadot/Kusama signed extension? Or that's from another chain?

[clangenb]

It is on Kusama since today, runtime release v1.2.5, and on all system chains. It will come to polkadot with the next runtime upgrade.

[leonardocustodio]

This has been completed per #516