Also expose HstRequestContext through DefineObjectsTag

I am not sure how it is currently done, but I guess having the hstRequestContext available through easy access in a jsp/ftl also makes sense : For example in case you want to access requestContext.getContentBean. Although you can normally access the requestContext through the HstRequest, the latter can be null in case of none-component-aggregated rendering. The HstRequestContext in that case is still available