More efficient copy object (#6665)

* it compiles

* refactored

* reduce to 4 concurrent chunk upload

* CopyObjectPartHandler

* copy a range of the chunk data, fix offset size in copied chunks

* Update s3api_object_handlers_copy.go

What the PR Accomplishes:
CopyObjectHandler - Now copies entire objects by copying chunks individually instead of downloading/uploading the entire file
CopyObjectPartHandler - Handles copying parts of objects for multipart uploads by copying only the relevant chunk portions
Efficient Chunk Copying - Uses direct chunk-to-chunk copying with proper volume assignment and concurrent processing (limited to 4 concurrent operations)
Range Support - Properly handles range-based copying for partial object copies

* fix compilation

* fix part destination

* handling small objects

* use mkFile

* copy to existing file or part

* add testing tools

* adjust tests

* fix chunk lookup

* refactoring

* fix TestObjectCopyRetainingMetadata

* ensure bucket name not conflicting

* fix conditional copying tests

* remove debug messages

* add custom s3 copy tests

weed/s3api/s3_constants/header.go

@@ -51,6 +51,12 @@ const (
 	AmzAclReadAcp     = "X-Amz-Grant-Read-Acp"
 	AmzAclWriteAcp    = "X-Amz-Grant-Write-Acp"
 
+	// S3 conditional copy headers
+	AmzCopySourceIfMatch           = "X-Amz-Copy-Source-If-Match"
+	AmzCopySourceIfNoneMatch       = "X-Amz-Copy-Source-If-None-Match"
+	AmzCopySourceIfModifiedSince   = "X-Amz-Copy-Source-If-Modified-Since"
+	AmzCopySourceIfUnmodifiedSince = "X-Amz-Copy-Source-If-Unmodified-Since"
+
 	AmzMpPartsCount = "X-Amz-Mp-Parts-Count"
 )