Store object EC in metadata header (#19534)

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.

cmd/metacache-entries.go

@@ -120,6 +120,16 @@ func (e *metaCacheEntry) matches(other *metaCacheEntry, strict bool) (prefer *me
 	for i, eVer := range eVers.versions {
 		oVer := oVers.versions[i]
 		if eVer.header != oVer.header {
+			if eVer.header.hasEC() != oVer.header.hasEC() {
+				// One version has EC and the other doesn't - may have been written later.
+				// Compare without considering EC.
+				a, b := eVer.header, oVer.header
+				a.EcN, a.EcM = 0, 0
+				b.EcN, b.EcM = 0, 0
+				if a == b {
+					continue
+				}
+			}
 			if !strict && eVer.header.matchesNotStrict(oVer.header) {
 				if prefer == nil {
 					if eVer.header.sortsBefore(oVer.header) {