Add support for preservation of unknown properties

[aseovic]

At the moment, Genson either silently skips over JSON properties that do not have a corresponding JavaBean property (default), or throws an exception if failOnMissingProperty is set to true.

In some cases it is important and necessary to support preservation of unknown properties. For example, this allows clients and the server to have different versions of the JavaBean classes (typically, the server has the latest version with a full property set, and clients can either have the latest or one of the older versions) without losing properties that are not known to all clients and the servers as JSON documents move through the system.

Jackson supports this via @JsonAnySetter and @JsonAnyGetter annotations, but I believe a better approach would be to support unknown property handling in a generic way, and the users can decide what the UknownPropertyHandler should do during serialization and deserialization.

I have already implemented a proposal and will submit a pull request shortly, but just wanted to open this issue to provide some context for the PR.

[EugenCepoi]

Perfect. This makes sense. Will take a look at the PR.