Replace ForSqlServerHasIndex with HasIndex

[AndriySvyryd]

See #4846

[divega]

PostgreSQL 11 now also supports INCLUDE in CREATE INDEX. It seems to work just like the SQL Server feature.

Just posting it here in case we want to revisit our previous decisions around this API.

[khellang]

Support is being added to Npgsql.EntityFrameworkCore.PostgreSQL here; npgsql/efcore.pg#699