Add attachments to problem object

[nickygerritsen]

Problem attachments (often testing tools for interactive problems) are currently not part of the spec (they are in the problem package but teams can't access that of course). Given that we do expose the statement, I think it makes sense to also expose attachments.

[nickygerritsen]

Do we do one ZIP with all attachments or an array with one attachment per item? DOMjudge/domjudge#3057 implements the latter:

[nickygerritsen]

From Baku discussion: no zip.

I'll work on this