Fix sub-inheritance mapping attributes generation

[tarasverq]

Hello.

This PR fixes generation of InheritanceMappingAttribute when you have stronger hierarchy than just one child.

Example

Let's pretend we have such class hierarchy:

public class WithInheritance
{
  public int Id { get; set; }
  public string Discriminator { get; set; } = null!;
}

public class WithInheritanceA : WithInheritance { }

public class WithInheritanceA1 : WithInheritanceA { }

public class WithInheritanceA2 : WithInheritanceA { }

Then when you try to get all WithInheritanceA objects linq2db would create this query:

SELECT
  t1."Id",
  t1."Discriminator"
FROM
  "WithInheritance" t1
WHERE
  t1."Discriminator" = 'WithInheritanceA'

But that's wrong: inherited entities WithInheritanceA1 and WithInheritanceA2 also should be included in query.

(t1."Discriminator" = 'WithInheritanceA'  OR
 t1."Discriminator" = 'WithInheritanceA1' OR 
 t1."Discriminator" = 'WithInheritanceA2')

My PR fixes that problem by finding all relations between inherited types and then adding it to collection of mapping attributes. Unit test also included, was tested on PgSql

[MaceWindu]

Thanks!