Add optional geography_type field to Geography spec

[marie-x]

Is your feature request related to a problem? Please describe.

Per 1.1 Working Group meetings, the idea of clarifying Geography objects via free-form types in an optional geography_type was raised as a useful non-breaking addition.

Describe the solution you'd like

Add an optional string field to the Geography object definition

Is this a breaking change

• No, not breaking

Impacted Spec

For which spec is this feature being requested?

• geography

Describe alternatives you've considered

We thought about not adding it ... another proposal was to add a list of geography_types when a geography could plausibly be more than one type of area, but so far nobody has identified clear and useful examples of this.

[schnuerle]

Added in #582