Format agnostic serializers

[sargunv]

The focus of this library is on GeoJSON, which is JSON, but ideally our serializers should be written to not assume JsonDecoder in order to be able to round trip other encodings in some reasonable way.

TODO:

• polymorphic serializers (Geometry, GeoJsonObject) rely on JsonContentPolymorphicSerializer
  • fix: complex. see comments in Generic feature properties #235 (comment)
• Feature.properties relies on JsonObject
  • fix: use a generic properties type: Generic feature properties #235

[sargunv]

#267 adds some additional tests for non-JSON serialization. Findings:

• CBOR works well with all concrete types (of course not polymorphism yet, because of JsonContentPolymorphicSerializer)
• Protobuf also works only if bbox is not null. Protobuf doesn't support nullable collections, so we'll need to adjust the BoundingBox serializer to support this use case if desired.