net: pkt: Added new function net_pkt_append_memset() to prefill packet

Some locations like DHCPv4 client create a prefilled packet by appending
new fragments in a loop with one byte each via net_pkt_append_u8() which
is wasteful and noisy. This patch adds the new functions
net_pkt_append_memset() which creates fragments as needed in the desired
size and initialises it to the specified value.

This change also adds a unittest for the new function.

Prerequisite for #9287

Signed-off-by: Daniel Egger <daniel@eggers-club.de>