Add field seasonal to drinking water

[paulklie]

Description, Motivation & Context

seasonal is used to mark if a drinking water amenity is turned off for the winter.

Links and data

https://wiki.openstreetmap.org/wiki/Tag:amenity%3Ddrinking_water
https://wiki.openstreetmap.org/wiki/Key:seasonal

Relevant tag usage stats:
Currently 4 675 drinking waters have seasonal set. That is 1.35% of all elements.

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[paulklie]

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