[Protocol Specs] Metadata specs are not up-to-date with demo examples

[mmv08]

The struct for plugin metadata from the demo doesn't match the specs:
https://github.com/5afe/safe-core-protocol-demo/blob/main/contracts/contracts/Base.sol#L18-L24

(There's a todo to add more examples, but not a todo to update the actual specs, so I'm creating this)