provide directory for additional data files for reports

[pirat89]

Sometimes you would want to add info into the report file

• which is not formated well in html and then report looks pretty messy,
• it has just little informational value
• ....

For that reason, would be great have some place, where the file with these additional data will be stored
and will be part of the tarball. Currently doesn't exist such place.

==== just minor notices, without effect on text above ====
Notice, that new openscap split stderr & stdout. So for some kind of additional data we can
print such info into the stdout, which is currently not used for anything and now mainly will serve for
us for quick review that probably some mess is printed outside. Previously this waste could be lost between logs. But it could be used for such content in some cases too (at least, it resolves first bullet from list above of the problem). But still, the directory should exists and should be provided in API.
( @AloisMahdal what do you think about such use case? ^ - not sure if you have already prepared some test for wate in stdout now. So I ask you too, for your opinion.)

Notice2, currently still some standardized directories are not provided in API. (not sure whether it is prepared in some side branch now).

[AloisMahdal]

I think we could introduce "report attachments" concept.

attach_file SRC [DST]
attach_file /path/to/orig.file attached.filename

would store orig.file as attached.filename in a directory dedicated to the module. Then links in solution text would be based on this path eg. see attached [diff](attached.diff).

What do you think?

[AloisMahdal]

Note that this is similar to hooks in that hooks are a special type of attachments (have "type" and auto-executed).

[AloisMahdal]

See PoC version at my repo:

https://github.com/AloisMahdal/preupgrade-assistant/commit/265adcc74314aa958efa4ac8f311fe5186b27026