Add addr:milestone support to Nominatim

[matheusgomesms]

I would like to propose support of addr:milestone from Nominatim: https://wiki.openstreetmap.org/wiki/Key:addr:milestone

It is a way to tag places that are usually in rural areas, by some highway, and have no official addr:housenumber, but use it as a proper address. It is usually stated in a sign/website (and even used by postal letters). Examples:

Highway A-1, km 12 (fictitious)
Rodovia BR-116, km 16.5 (a place that advertises in their website this address, and uses it for postal purposes).

The proposed tagging can be seen in the last OSM link, and Nominatim probably should handle it the same way as it does with addr:housenumber, but also appending 'km' before the addr:milestone value, if it is not in miles.

Rationale for this request: this happens in diverse parts of the world, but very likely people don't use it in OSM for many reasons (not officially approved, lack of support by search engines and apps etc), classic egg/chicken problem that happens in OSM all the time.

In Brazil, we are working towards a full national import of addresses, which contains millions of addresses, including several ones like this. So we are probably picking this tag to be used for those cases, and would like to see Nominatim supporting it.

Finally, we saw that there are at least 4500 cases, worldwide, that have 'km' (or similar) in addr:housenumber, and could benefit from this tagging.

addr:milestone in Taginfo: https://taginfo.openstreetmap.org/keys/addr:milestone#overview

[matheusgomesms]

PS: also probably related to #69