Support entity type members defined through default interface implementation

### What problem are you trying to solve?

First of all thanks for your work, efcore is very powerful!

If you define indexer properties only in the interface of the entity implementation, the ModelBuilder IndexerProperty method will report an error.
``` csharp
public interface IHasIndexerProperties
{
protected virtual Dictionary Data=>new Dictionary();
public virtual object this[string key]
{
get=>Data[key];
set=>Data[key]=value;
}
}
public class Blog:IHasIndexerProperties
{
public string Url{get;set;}
}

modelBuilder.Entity().IndexerProperty("VisitCount"); // exception occurred
```

### Describe the solution you'd like

_No response_