Keep the EC in header, so it can be retrieved easily for dynamic quorum calculations.
To not force a full metadata decode on every read the value will be 0/0 for data written in previous versions.
Size is expected to increase by 2 bytes per version, since all valid values can be represented with 1 byte each.
Example:
```
λ xl-meta xl.meta
{
"Versions": [
{
"Header": {
"EcM": 4,
"EcN": 8,
"Flags": 6,
"ModTime": "2024-04-17T11:46:25.325613+02:00",
"Signature": "0a409875",
"Type": 1,
"VersionID": "8e03504e11234957b2727bc53eda0d55"
},
...
```
Not used for operations yet.