etag: add FromContentMD5 to parse content-md5 as ETag (#11688)

This commit adds the `FromContentMD5` function to
parse a client-provided content-md5 as ETag.

Further, it also adds multipart ETag computation
for future needs.

cmd/utils.go

@@ -20,7 +20,6 @@ import (
 	"bytes"
 	"context"
 	"crypto/tls"
-	"encoding/base64"
 	"encoding/json"
 	"encoding/xml"
 	"errors"
@@ -143,18 +142,6 @@ func xmlDecoder(body io.Reader, v interface{}, size int64) error {
 	return d.Decode(v)
 }
 
-// checkValidMD5 - verify if valid md5, returns md5 in bytes.
-func checkValidMD5(h http.Header) ([]byte, error) {
-	md5B64, ok := h[xhttp.ContentMD5]
-	if ok {
-		if md5B64[0] == "" {
-			return nil, fmt.Errorf("Content-Md5 header set to empty value")
-		}
-		return base64.StdEncoding.Strict().DecodeString(md5B64[0])
-	}
-	return []byte{}, nil
-}
-
 // hasContentMD5 returns true if Content-MD5 header is set.
 func hasContentMD5(h http.Header) bool {
 	_, ok := h[xhttp.ContentMD5]