improve custom serialization support in subdoc

Subdocument's MutateInBuilder and LookupInBuilder both can be configured with a FragmentTranscoder, but it can only be set when manually constructing the instances.

Additionally, the only implementation currently is based on the shaded version of Jackson, which implies that it cannot be constructed with a client-provided ObjectMapper (unless they instantiate a shaded one) nor will it consider client-side Jackson annotations like @JsonIgnore...

Third issue is that the LookupInBuilder doesn't allow to perform a get with a specific class, leading to the FragmentTranscoder deserializing fragments as generic Object like a Map (or a JsonObject in the case of the default implementation).