Changes to `EntityValidateRelations`

• Fix doc comments

• Add IgnoreEmpty property (default to true)

• Make sure that the stream can still be read

• Add alias ValidateRelations

• Change the aliases and documentation

• 
    /// <summary>
  
    /// The entity property which will be used to create a lookup table.
  
    /// </summary>
  
    [StepProperty(2)]
  
    [Required]
  
    [Alias("ExistsIn")]
  
    public IStep<StringStream> IdProperty { get; set; } = null!;
  
  
  
    /// <summary>
  
    /// The step will check that the value of this entity property exists in
  
    /// the lookup table.
  
    /// </summary>
  
    [StepProperty(3)]
  
    [Required]
  
    [Alias("LookupThat")]
  
    public IStep<StringStream> ParentIdProperty { get; set; } = null!;```