Allow embedded indexing

This functionality allows creating of one document for search from two embedded index.

Example:

```java
@SearchIndex(name = "AddressIndex")
public class AddressIndex {

  @SearchId
  private Long entityId;

  @FullTextSearch
  private String city;

  private String postCode;

}

```

and index

```java
@SearchIndex(name = "CustomerIndex")
public class CustomerIndex {

  @SearchId
  private Long entityId;

  private String customerName;

  private AddressIndex address;
}
```

will be represented in one search document in format

Document {
entityId
customerName
address_entityId
address_city
address_postCode
}