Add the metadata hash signed extension to the extrinsic definition

[IkerAlus]

As of block 21,455,000, Polkadot runtime version 1.2.5 is live in the relay chain. With it, the extrinsic format is changed to add the possibility to check the metadata hash as per RFC 78. Hence, Definition 155 and Definition 156 of the Polkadot Protocol Specification must be updated accordingly.

[bhargavbh]

thanks for flagging. we are looking to update the whole Extrinsics chapter as quite a few protocol changes are not reflected in the Spec