when compiling using GCC-13, it warns that:
```
/home/kefu/dev/scylladb/utils/s3/client.cc:224:9: error: stack usage might be 66352 bytes [-Werror=stack-usage=]
224 | sstring parse_multipart_upload_id(sstring& body) {
| ^~~~~~~~~~~~~~~~~~~~~~~~~
```
so it turns out that `rapidxml::xml_document<>` could be very large,
let's allocate it on heap instead of on the stack to address this issue.
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closes#13722
37f1beade5