aggregate_key() and aggregate_index()

[mitchelloharawild]

Mixed-granularity data analysis is not specific to forecasting, and so these functions are better placed in the {tsibble} package (rather than {fabletools}. The fabletools package has provided aggregate_key() for a while, although it has never released a version with aggregate_index(). The aggregate_index() function will be powered by the mixed granularity temporal manipulation tools in {mixtime}.