Allow alternative join types for Flows

[jc-harrison]

Currently, Flows queries are hard-coded to use an inner join. There are situations in which we would like to include counts of subscribers per location in 'loc1' who do not appear in 'loc2' and vice versa (i.e. 'disappeared' and 'appeared' groups), which could be achieved by using an outer join in a Flows query.

I suggest we expose the how parameter of the internally-constructed Join as a parameter of Flows itself, to enable this use case.