wip: spin:geo SIP

[lann]

Cloud services sometimes care about certain geographic information:

• The service host's physical location and/or data center information for
  telemetry or network route selection
• The approximate geographic location of a client for e.g providing localized
  responses

Separately, a local Spin app may be interested in the user's physical location
as provided by their device (e.g. GPS or network-based geolocation).