Allow nested request body inside @BeanParam

That would be a brilliant aggregate
```
@Value
@Builder(toBuilder = true)
@NoArgsConstructor(force = true)
@AllArgsConstructor
public class Changes {

@Context
Locale locale;

@HeaderParam("Authorization")
String token;

@QueryParam("rewrite")
String rewrite;

@PathParam("id")
String id;

@JsonProperty
@NotNull
@Valid
AddressDTO three;

}
```
Now AddressDTO is null, of course

so instead,
```
@POST
UserDTO dostuff(@BeanParam Changes changes, @NotNull @Valid UserDTO)
```
you could just
```
@POST
UserDTO dostuff(@BeanParam Changes changes)
```
and of course deep nesting would allow even more.
Especially with some libraries like mapstruct, where all that hierarchy automatically would be remapped to any other bunch of objects with possibly completely different topology. This would allow very convenient transitioning from web-specific layer data to , say, db/persistence layer.

Start by tracing how @BeanParam currently binds fields and how the nested AddressDTO in the example is treated. Define the behavior for nested request-body binding, including deep nesting, and verify that the example can be represented through a single @BeanParam Changes argument.

Written by the indexing model from the issue text.