Add "other" `CEconItemDefinition` members natives

## Add natives for the following members:
- [ ] `m_iInventoryImageSize`
- [x] `m_iItemSets` #15 
- [ ] `m_pTool`
- [ ] `m_PaintData`
- [ ] `m_pInventoryImageData`
- [ ] `m_vecTags`
- [ ] `m_vecContainingBundleItemDefs`
- [x] `m_vecSteamWorkshopContributors` #16 
- [x] `m_vecStaticAttributes` #14