Add more hiking-specific POIs

[kylebarron]

Mostly taken from https://wiki.openstreetmap.org/wiki/Hiking

Include shelter_type?

Infrastructure:

• tourism=picnic_site already exists: https://github.com/openmaptiles/openmaptiles/blob/7678fefe436cd92c6088d789efa911e63d074d99/layers/poi/mapping.yaml#L308
• barrier=gate already exists (but is not rendered) https://github.com/openmaptiles/openmaptiles/blob/7678fefe436cd92c6088d789efa911e63d074d99/layers/poi/mapping.yaml#L66-L74
• information=guidepost

Nature:

• natural=peak exists as mountain_peak openmaptiles layer
• natural=saddle, done in f21f596. Maybe use the viewpoint maki icon for this.
• mountain_pass=yes, probably just use natural=saddle.
• natural=ridge
• natural=cliff
• tourism=viewpoint exists and is set to attraction: https://github.com/openmaptiles/openmaptiles/blob/bf833dcd4577811ff9c002ecc9f6e99c0ad43709/layers/poi/class.sql#L58
• natural=spring nst-guide/osm-liberty-topo@f1f75e9
• natural=drinking_water nst-guide/osm-liberty-topo@f1f75e9