feat: decommission feature for pools (#14012)

```
λ mc admin decommission start alias/ http://minio{1...2}/data{1...4}
```

```
λ mc admin decommission status alias/
┌─────┬─────────────────────────────────┬──────────────────────────────────┬────────┐
│ ID  │ Pools                           │ Capacity                         │ Status │
│ 1st │ http://minio{1...2}/data{1...4} │ 439 GiB (used) / 561 GiB (total) │ Active │
│ 2nd │ http://minio{3...4}/data{1...4} │ 329 GiB (used) / 421 GiB (total) │ Active │
└─────┴─────────────────────────────────┴──────────────────────────────────┴────────┘
```

```
λ mc admin decommission status alias/ http://minio{1...2}/data{1...4}
Progress: ===================> [1GiB/sec] [15%] [4TiB/50TiB]
Time Remaining: 4 hours (started 3 hours ago)
```

```
λ mc admin decommission status alias/ http://minio{1...2}/data{1...4}
ERROR: This pool is not scheduled for decommissioning currently.
```

```
λ mc admin decommission cancel alias/
┌─────┬─────────────────────────────────┬──────────────────────────────────┬──────────┐
│ ID  │ Pools                           │ Capacity                         │ Status   │
│ 1st │ http://minio{1...2}/data{1...4} │ 439 GiB (used) / 561 GiB (total) │ Draining │
└─────┴─────────────────────────────────┴──────────────────────────────────┴──────────┘
```

> NOTE: Canceled decommission will not make the pool active again, since we might have
> Potentially partial duplicate content on the other pools, to avoid this scenario be
> very sure to start decommissioning as a planned activity.

```
λ mc admin decommission cancel alias/ http://minio{1...2}/data{1...4}
┌─────┬─────────────────────────────────┬──────────────────────────────────┬────────────────────┐
│ ID  │ Pools                           │ Capacity                         │ Status             │
│ 1st │ http://minio{1...2}/data{1...4} │ 439 GiB (used) / 561 GiB (total) │ Draining(Canceled) │
└─────┴─────────────────────────────────┴──────────────────────────────────┴────────────────────┘
```

cmd/storage-datatypes.go

@@ -72,11 +72,12 @@ type FilesInfo struct {
 	IsTruncated bool
 }
 
-// FilesInfoVersions represents a list of file versions,
-// additionally indicates if the list is last.
-type FilesInfoVersions struct {
-	FilesVersions []FileInfoVersions
-	IsTruncated   bool
+// Size returns size of all versions for the object 'Name'
+func (f FileInfoVersions) Size() (size int64) {
+	for _, v := range f.Versions {
+		size += v.Size
+	}
+	return size
 }
 
 // FileInfoVersions represent a list of versions for a given file.
@@ -235,7 +236,9 @@ func (fi *FileInfo) SetInlineData() {
 }
 
 // VersionPurgeStatusKey denotes purge status in metadata
-const VersionPurgeStatusKey = "purgestatus"
+const (
+	VersionPurgeStatusKey = ReservedMetadataPrefixLower + "purgestatus"
+)
 
 // newFileInfo - initializes new FileInfo, allocates a fresh erasure info.
 func newFileInfo(object string, dataBlocks, parityBlocks int) (fi FileInfo) {