We do not present a validated timestamp in challenges where status = valid
as required by RFC8555.
This change is the first step to presenting challenge timestamps to the
client. It adds a timestamp to each place where we change a challenge to
valid. This only displays in the logs and will not display to the
subscriber because it is not yet stored somewhere retrievable. The next
step will be to store it in the database and then finally present it to
the client.
Part of #5198
aa20bcaded