fix: dev: Fix PROXYv2 header size truncation bug

The 'isc_proxy2_handler_t' structure stores some size values in a
'uint16_t' type, while the maximum size can be bigger, which results
in truncation. Change the affected types to 'size_t'.

Closes #6142

Merge branch '6142-proxyv2-header-length-fix' into 'main'

See merge request isc-projects/bind9!12294