Add second-level administrative division to PostalAddress

[FilippoVigani]

(Duplicate of schemaorg/schemaorg#3414)

Currently addressRegion should be used for the "region" in which the locality of an address is. For the United States, it's the state, for other countries it should contain a first-level administrative division, as specified by its description:

The region in which the locality is, and which is in the country. For example, California or another appropriate first-level Administrative division.

Often it is required to store information about second-level administrative division, for instance a county in the US or a Provincia in Italy. We currently lack a term for that in the schema.org dictionary.

As for third-level administrative division, usually it encompasses cities, towns, etc, so I imagine the proper term to use is addressLocality, which makes it very clear that we are lacking a term for the second-level.

I'm not sure what term would be appropriate, but to be fair using addressRegion as first-level administrative division isn't very clear, as technically there are countries where their "regions" are considered second-level divisions. At the same time using something like addressProvince for second-level divisions isn't great, as there are many countries where a "province" is the first-level. Maybe we should steer away from such names and use "administrative divisions" with different levels, the same way Google APIs structure their data?