Add a `FsSql.Npgsql` package that registers `NpgSqlCommand` as supporting async ops

[baronfel]

Would need to be a new package so that we don't have Npgsql as a dependency of the main project.

[irium]

Maybe because of I'm not native english, but I don't understand what you mean. Currently there is no any dependency on Npgsql at all. If you mean to create new project and package - feel free to do it. I can mention it in the README if you want.

[baronfel]

Yep, that's all I mean. I ran into this today and mostly wanted to create an issue in case someone else had seen this project too! We're working on a package at work to do what I mentioned above and wehn we get something more solid I'll write back.