spdx30: Allow VEX Justification to be configurable

spdx30: Allow VEX Justification to be configurable

Instead of hard coding the VEX justifications for "Ignored" CVE status,
add a map that configures what justification should be used for each
status.

This allows other justifications to be easily added, and also ensures
that status fields added externally (by downstream) can set an
appropriate justification if necessary.

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>