Flatten closed-interface projection with nested entities

**Is your feature request related to a problem? Please describe.**

Subsequent ticket for https://github.com/GoogleCloudPlatform/spring-cloud-gcp/issues/2387.

When I use a projection to get nested entity with closed-interface, I expect spring-cloud-gcp-data to query specified fields only through the interface in the closed-interface.

For example)

if `LightCat.java` has
```
package com.example.projectiontest.models;

import java.time.Instant;

public interface LightCat {
Long getId();

String getName();
Breed getBreed();

Person getOwner();

Instant getFound();

interface Person {
String getName();
}
}
```

Currently, when I check `PartTreeDatastoreQuery.java`'s `getEntityOrProjectionQueryBuilder()`, it queries projection with the following fields: `id, name, breed, owner, found`

I expect the query to get `id, name, breed, owner.name, found`.

**Describe alternatives you've considered**
Having Datastore Converter for converting between `Person` and `String` would make it work, but if the `Person` object is complicated, it will have some overheads for collecting all data.

**Additional context**
Sample repository: https://github.com/k8kim/spring-gcp-data-projection-demo
Spring cloud gcp version: 4.8.3
Spring cloud version: 2022.0.4

In the repository, there is a rest controller (CatController.java), datastore repository (CatRepository), and some entities (Cat.java, LightCat.java, Person.java). LightCat is an interface for projecting Cat object.

The rest controller contains 4 endpoints, but the ones that are related here are:

{POST} /cats : add a cat with name and ownerName from request parameters and some default values.
{GET} /cats: returns a list of Cats in the datastore
{GET} /light-cats: returns a list of LightCats in the datastore

Start in PartTreeDatastoreQuery.java at getEntityOrProjectionQueryBuilder() and reproduce the LightCat projection using the sample repository's CatController and related entities. Verify the /light-cats endpoint and datastore query behavior; done means nested closed-interface fields such as owner.name are selected instead of the whole nested entity.

Written by the indexing model from the issue text.