Support for `partial_match` attributes on Google geocoding

https://developers.google.com/maps/documentation/geocoding/intro

> `partial_match` indicates that the geocoder did not return an exact match for the original request, though it was able to match part of the requested address. You may wish to examine the original request for misspellings and/or an incomplete address.

> Partial matches most often occur for street addresses that do not exist within the locality you pass in the request. Partial matches may also be returned when a request matches two or more locations in the same locality. For example, "21 Henr St, Bristol, UK" will return a partial match for both Henry Street and Henrietta Street. Note that if a request includes a misspelled address component, the geocoding service may suggest an alternative address. Suggestions triggered in this way will also be marked as a partial match.

We should label output with a `partial_match` attribute that can be extracted programmatically. It shouldn't change existing workflows, hence the use of `attr` instead of something like an additional field in the output.

cc: @j-d-miller