ext/curl: Add `CURLINFO_POSTTRANSFER_TIME_T` support (GH-15849)

libcurl ref: [`CURLINFO_POSTTRANSFER_TIME_T`](https://curl.se/libcurl/c/CURLINFO_POSTTRANSFER_TIME_T.html)

`CURLINFO_

ext/curl: Add `CURLINFO_POSTTRANSFER_TIME_T` support (GH-15849)

libcurl ref: [`CURLINFO_POSTTRANSFER_TIME_T`](https://curl.se/libcurl/c/CURLINFO_POSTTRANSFER_TIME_T.html)

`CURLINFO_POSTTRANSFER_TIME_T` is a libcurl info option that returns
the time it took to "post" the transfer. Available since libcurl 8.10.0

This value is also exposed as `posttransfer_time_us` in the
`curl_getinfo()` return value when the `$option` parameter is not
passed.

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