s3: validate per-part checksums in CompleteMultipartUpload

Parse the Checksum* elements of each completed part and enforce what AWS
does for uploads created with x-amz-checksum-algorithm: every part must
carry a checksum in the complete request (InvalidRequest when missing,
BadDigest when it differs from the stored part checksum), and an
x-amz-checksum-type header must match the upload resolved checksum type
(BadDigest). Add the issue-11401 reproduction as a regression test.

weed/s3api/s3api_object_handlers_multipart.go

@@ -645,6 +645,7 @@ type CompleteMultipartUpload struct {
 type CompletedPart struct {
 	ETag       string
 	PartNumber int
+	ChecksumResult
 }
 
 // handleSSES3MultipartHeaders handles SSE-S3 multipart upload header setup to reduce nesting complexity