Full support of per-request overlays in updating operations

[andrus]

Currently, update requests only partially support per-request overlays. It is possible to redefine read / write policies of the root entity properties, but the following doesn't work:

• request parsing happens in the context of the common schema, so produced EntityUpdates do not reference overlaid AgEntities
• nested overlays are ignored (unless they are included in the read ResourceEntity), making it impossible to implement tasks like Filter nested unwritable attributes from updates #637

A few things we can do to handle this properly:

• Model request overlays as a single object that can be shared between read and write code (e.g. RequestSchema)
• (a big breaking change !!, but with meaningful deprecation) Make EntityUpdate more "opaque", with only parsed JSON and root Java class, but no attribute / relationship classification. For backwards compatibility deprecate EntityUpdate, but keep supporting it. Instead recommend binding updates to a new opaque ObjectUpdate object that only contains the declared entity type and a JSON blob. This way overlays can be defined before parsing.
• Update OpenAPI adapters to handle ObjectUpdate.