curl_ngtcp2: extend callback tables for nghttp3 1.11.0 and ngtcp2 1.14.0

curl_ngtcp2: extend callback tables for nghttp3 1.11.0 and ngtcp2 1.14.0

An emergency update to initialize callbacks to NULL.

May need further updates, e.g. to provide the recommended random
generator callback for nghttp3.

Fixing potential crashes at runtime in curl-for-win 8.15.0_3, and these
build warnings:
```
/home/appveyor/projects/curl-for-win/curl/lib/vquic/curl_ngtcp2.c:836:1:
warning: missing field 'begin_path_validation' initializer [-Wmissing-field-initializers]
  836 | };
      | ^
/home/appveyor/projects/curl-for-win/curl/lib/vquic/curl_ngtcp2.c:1186:1:
warning: missing field 'recv_origin' initializer [-Wmissing-field-initializers]
 1186 | };
      | ^
2 warnings generated.
```
Ref: https://ci.appveyor.com/project/curlorg/curl-for-win/builds/52462852#L14821

Also causing all `GHA/curl-for-win` jobs to fail:
https://github.com/curl/curl/actions/runs/16523625082?pr=18010

Ref: https://github.com/ngtcp2/nghttp3/releases/tag/v1.11.0
Ref: https://github.com/ngtcp2/ngtcp2/releases/tag/v1.14.0
Ref: https://github.com/curl/curl-for-win/commit/ff788c81e44f998d0d41f362d442a7e97cc0fd46

Closes #18019