Changes:
- add new top level interface `Plugin`
- add new `JsonDeserializer` aware of the `PluginRegistry`
- add new annotation processor for plugin
- migrate to hibernate-validator for the JSR-303 (no bean introspection dependencies)
- update all Validation annotations to be complitant with the JSR-303
- remove all KestraApplicationContext stuffs
- fix some tests
part-of: kestra-io/plugin-template-test#25
part-of: kestra-io/plugin-serdes#103