fbt: Add support for CTFv3 containers

The general aim in this and subsequent patches is to minimize the
amount of code that directly references CTF types such as ctf_type_t,
ctf_array_t, etc. To that end, introduce some routines similar to the
existing fbt_get_ctt_size() (which exists to deal with differences
between v1 and v2) and change ctf_lookup_by_id() to return a void
pointer.

Support for v2 containers is preserved.

MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34361