Add Table.AddKey Functionality to UI to Optimise Queries

[alex_holloway]

It seems as though there is a performance benefit to query runtime when manually adding keys which currently has to be done in M (see: https://blog.crossjoin.co.uk/2018/03/16/improving-the-performance-of-aggregation-after-a-merge-in-power-bi-and-excel-power-query-gettransform/) This may be particularly true when creating a join on a csv, for example, where a foreign key may vastly improve performance. Please allow us to do this in the UI rather than needing to modify the M behind a table