Provide API for returning Func<TEntity, bool> predicated based on keys

[Liero]

Problems I'm trying to solve

In short I want to do this:

object[] keys = ....;
dbContext.Set<TEntity>().Local.Find(keys)
//or
dbContext.ChangeTracker.Entries<TEntity>().Find(keys);

or

bool EqualsByPK(TEntity entity1,  TEntity entity2);

or

resolve duplicates as described in the change-tracking/identity-resolution docs.

In other words, find out, whether entity with specified key is already tracked:

   context.ChangeTracker.TrackGraph(entity, node =>
        {
            var entityType = node.Entry.Metadata;                
                            
            //documented way is not generic because of hard code Id property
            //var keyValue = node.Entry.Property("Id").CurrentValue; //hard coded ID
            //var existingEntity = node.Entry.Context.ChangeTracker.Entries()
            //    .FirstOrDefault(
            //        e => Equals(e.Metadata, entityType)
            //             && Equals(e.Property("Id").CurrentValue, keyValue)); //hard coded ID
            
            object[] keyValues = entityType.GetKeyValues(node.Entry.Entity); //new api
            //object[] keyValues = entityType.GetKeys()
            //    .FirstOrDefault(key => key.IsPrimaryKey())?.Properties
            //    .Select(property => node.Entry.CurrentValues[property])
            //    .ToArray();
              

            var predicate = entityType.CreatePrimaryKeyPredicate(keyValues); //new API
                .dbContext.ChangeTracker.Entries().Where(e => Equals(e.Metadata, entityType))
                .FirstOrDefault(e => predicate(e.Entity));

            var existingEntity = dbContext.ChangeTracker.Entries().FirstOrDefault(predicate);
                
            if (existingEntity == null)
            {
                Console.WriteLine($"Tracking {entityType.DisplayName()} entity with key value {keyValue}");

                node.Entry.State = EntityState.Modified;
            }
            else
            {
                Console.WriteLine($"Discarding duplicate {entityType.DisplayName()} entity with key value {keyValue}");
            }
        });

Proposed solution:

new APIs:

1. object[] GetKeyValues(this IEntityType entityType, object Entity);

2. Func<bool, object>(this IEntityType entityType, entityType.CreatePrimaryKeyPredicate(keyValues))

   object[] keyValues = entityType.GetKeyValues(node.Entry.Entity);
   //object[] keyValues = entityType.GetKeys()
   // .FirstOrDefault(key => key.IsPrimaryKey())?.Properties
   // .Select(property => node.Entry.CurrentValues[property])
   // .ToArray();
   var predicate = entityType.CreatePrimaryKeyPredicate(keyValues);
   dbContext.ChangeTracker.Entries().Where(e => Equals(e.Metadata, entityType))
   .FirstOrDefault(predicate);

[ajcvickers]

Duplicate of #7391.