This pull request adds support of "NULLS NOT DISTINCT" option for PostgreSQL.

By default, unique indexes in PostgreSQL allow multiple null values (so, null values are considered not equal to each other). In PostgreSQL 15, new option was added: "NULLS NOT DISTINCT", which allows only single null value.
https://www.postgresql.org/docs/current/sql-createindex.html

Before PostgreSQL 15, there was a workaround with two indexes: https://stackoverflow.com/questions/8289100/create-unique-constraint-with-null-columns

To enable "NULLS NOT DISTINCT" for index, UniqueNullsNotDistinct method should be called:

Create.Index("MyIndex").OnTable("MyTable1").InSchema("public")
   .OnColumn("Column1").Ascending()
   .OnColumn("Column2").Ascending()
   .WithOptions().UniqueNullsNotDistinct();

Runner should be configured with AddPostgres15_0:

services
   .AddFluentMigratorCore()
   .ConfigureRunner(builder => builder.AddPostgres15_0());