Add a GPS Coordinate url to support resource validation

[sharon2719]

Description

There is a need to create a new URL to support the validation of resources. Currently we are using this

"extension": [
            {
              "url": "gps-coordinate",
              "valueString": "longitude"
            }
          ]

This fails the FHIR validation check with
Questionnaire.item[0].item[1].extension[0] - Extension.url must be an absolute URL

cc @dubdabasoduba

[dubdabasoduba]

@jingtang10 We are adding strict validation of our Questionnaires and StructureMaps. The Questionnaires using the GPS widget fail because the url on the GPS coordinates is not a URL.

We would like to update it to https://github.com/google/android-fhir/gps-coordinate