fix: enforce 5gb copy source object size threshold.

Fixes #1896

Enforces the S3 `5 GiB` copy source size limit across the posix and azure
backends for `CopyObject` and `UploadPartCopy`, returning `InvalidRequest` when
the source object exceeds the threshold.

The limit is now configurable via `--copy-object-threshold`
(`VGW_COPY_OBJECT_THRESHOLD`, default 5 GiB).
A new `--mp-max-parts flag` (`VGW_MP_MAX_PARTS`, default `10000`) has been added to make multipart upload parts number limit configurable.

No integration test has been added, as GitHub Actions cannot reliably
handle large objects.

s3err/s3err.go

@@ -1088,3 +1088,11 @@ func GetNegativeMaxLimiterErr(limiter string) APIError {
 		HTTPStatusCode: http.StatusBadRequest,
 	}
 }
+
+func GetCopySourceObjectTooLargeErr(limit int64) APIError {
+	return APIError{
+		Code:           "InvalidRequest",
+		Description:    fmt.Sprintf("The specified copy source is larger than the maximum allowable size for a copy source: %d", limit),
+		HTTPStatusCode: http.StatusBadRequest,
+	}
+}