Expose graph version and issued as well as modified times #604
Description
We'll probably want to rename the graph, but it would be useful to have access to the version and issued alongside the public graph modified times graph. (for e.g. Swirrl/ook#118)
We already track both internally, but at the level of draftsets/endpoints rather than at the level of graphs, so this will need to be changed to be consistent with the treatment of modified times.