net80211: add the beginning of the extfield information elements (IE ext)

The original list of IEs got expanded from TLV to TLextTV.
If the T matches 255 then we have a second list of IEs where the
meaning of TL stays the same. That means the 1 octet extT is part
of the length and the value starts at ie+3.

Start populating the list with IEEE802.11-2020 and 802.11ax-2021
values.

They will be initially used to start decoding some of the announced IEs
for ifconfig [-v] list (scan|sta). That should help users with
AX-enabled APs to see this (rather than no or UNKNOWN_ELEMID_255 and
make debugging easier once we implement 11ax.

Sposored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D50674