Add support for npgsql ILIKE operator

[snax4a]

This fixes an issue with case sensitive search in npgsql https://github.com/fullstackhero/dotnet-webapi-boilerplate/discussions/396

Here I implemented custom specification evaluator which uses the PostgreSQL ILIKE operator when Npgsql DBProvider is used.

I have tested it with PostgreSQL and MSSQL providers and it works correctly.

[snax4a]

@fretje I could not find a cleaner way of parametrizing the evaluator. Tell me if we can leave it in this way or if you have any idea how to clean it up.