fix(JSON): Resolve mapping issues

[hypery2k]

fixes #43

[flytreeleft]

@hypery2k The error Unrecognized field "xxx" should be disappeared if we use the latest version of the Keycloak SDK.

But, if we disable the DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES explicitly (JsonSerialization.mapper.disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES)), maybe the plugin can be compatible with the latest Keycloak without upgrading the Keycloak SDK anymore. So, I think the PR is worthy of merging.

Thanks @hypery2k :).

[hypery2k]

That was the idea behind it.
I compiled it with the older Keycloak SDK and tried it within a Keycloak 9 and seems to works.

[flytreeleft]

@hypery2k Great, thanks for your hard work :).