net: sockets: socketpair: support querying bytes available

In order to get a semi-accurate assessment of how many
bytes are available on a socket prior to performing a read,
BSD and POSIX systems have typically used

`ioctl(fd, FIONREAD, &avail)`

We can support this in Zephyr as well with little effort, so
add support for `socketpair()` sockets as an example.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>