Update `MetadataVersioned` to have `version` field

[HCastano]

Right now versioning in the ink! metadata is done through the use of a version key which
wraps the metadata object:

"V3": {
    ... ink! metadata object ...
}

We should instead change this to be better defined by introducing a seperate version
field into the ink! metadata.

"abi": {
  "version": "3",
  "spec": { ... },
  "storage": { ... },
  "types": { ... },
}

This makes the meaning of the field more clear. We'll want to introduce this along side
the V4 metadata changes.