Public API for registering custom value types

[andrus]

Currently to start supporting a new "value type" requires injection of two objects into the corresponding DI maps. E.g. here is how we added "Json" type support recently:

binder.bindMap(Encoder.class).put(Json.class.getName(), JsonEncoder.encoder());
binder.bindMap(JsonValueConverter.class).put(Json.class.getName(), JsonConverter.converter());

It is not obvious (even the naming of the custom objects is rather generic, so I had to wade though source code before I figured out what I need to extend). I think it would be beneficial to expose this API via an AgBuilder method, as this is a rather common operation.

While we are at it, we should probably move all the hardcoded default types to use these DI collections to have a single mechanism for collecting these converters.