add amplitude support for native region and city fields

Amplitude has [native fields](https://cloudup.com/c7V_z4BEdPN) for `city` and `region` that we should respect in addition to `country`, which we already do. This was brought up by an enterprise customer in ZD issue #19744.

@amillet89 @calvinfo 

question -- should we remove `traits.address` from user_properties after setting the native fields? i can update if needed. we had been keeping country in there in addition to the native field (https://github.com/segmentio/integrations/commit/fedfcb6037fee759bdc63f3022d90459a6e53f60#diff-28496b0c244325132bcab5031168d5d3L25), so i stuck with that pattern. definitely open to thoughts!