api: Introduce metadata update APIs to update only metadata (#11962)

Current implementation heavily relies on readAllFileInfo
but with the advent of xl.meta inlined with data, we cannot
easily avoid reading data when we are only interested is
updating metadata, this leads to invariably write
amplification during metadata updates, repeatedly reading
data when we are only interested in updating metadata.

This PR ensures that we implement a metadata only update
API at storage layer, that handles updates to metadata alone
for any given version - given the version is valid and
present.

This helps reduce the chattiness for following calls..

- PutObjectTags
- DeleteObjectTags
- PutObjectLegalHold
- PutObjectRetention
- ReplicateObject (updates metadata on replication status)

cmd/erasure-server-pool.go

@@ -1778,6 +1778,22 @@ func (z *erasureServerPools) Health(ctx context.Context, opts HealthOptions) Hea
 	}
 }
 
+// PutObjectMetadata - replace or add tags to an existing object
+func (z *erasureServerPools) PutObjectMetadata(ctx context.Context, bucket, object string, opts ObjectOptions) (ObjectInfo, error) {
+	object = encodeDirObject(object)
+	if z.SinglePool() {
+		return z.serverPools[0].PutObjectMetadata(ctx, bucket, object, opts)
+	}
+
+	// We don't know the size here set 1GiB atleast.
+	idx, err := z.getPoolIdxExisting(ctx, bucket, object)
+	if err != nil {
+		return ObjectInfo{}, err
+	}
+
+	return z.serverPools[idx].PutObjectMetadata(ctx, bucket, object, opts)
+}
+
 // PutObjectTags - replace or add tags to an existing object
 func (z *erasureServerPools) PutObjectTags(ctx context.Context, bucket, object string, tags string, opts ObjectOptions) (ObjectInfo, error) {
 	object = encodeDirObject(object)