When using runtime.Object and runtime.RawExtension, passing
runtime.Unknown to protobuf serializer should result in the raw message
being serialized (since all normal protobuf objects are
runtime.Unknown).
Also, avoid setting a content-type when decoding a protobuf object
except when the content appears to be proto.
The codec factory should support two distinct interfaces - negotiating
for a serializer with a client, vs reading or writing data to a storage
form (etcd, disk, etc). Make the EncodeForVersion and DecodeToVersion
methods only take Encoder and Decoder, and slight refactoring elsewhere.
In the storage factory, use a content type to control what serializer to
pick, and use the universal deserializer. This ensures that storage can
read JSON (which might be from older objects) while only writing
protobuf. Add exceptions for those resources that may not be able to
write to protobuf (specifically third party resources, but potentially
others in the future).
