Support format field in Compute Discovery Doc

[scotthart]

Compute is adding a format field to the JSON to accompany occurrences of "type": "any" in order to specify what the resulting protobuf type should be.

The generator should expect to see them together like:

"properties": {
        "field_name": {
            "type": "any",
            "format": "google.protobuf.Value"
        }
      }

This would result in a .proto field definition like:

  optional google.protobuf.Value field_name = <field number>;