Expose graph version and issued as well as modified times

[callum-oakley]

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.