Support for built-in PostgreSQL ranges

[roji]

Users can create arbitrary range types, but Npgsql's EF Core type mapper currently doesn't look at the database to construct the mappings (singleton), making it impossible to know the datatype name for a given NpgsqlRange. What we can do is support the built-in range types, which will probably be enough.

Includes scaffolding support.

[roji]

Scaffolding depends on dotnet/efcore#7269

[wub]

dotnet/efcore#7269 is done now - is there anything else blocking this?

Thanks for all the hard work, @roji. If it weren't for npgsql I probably wouldn't be using .NET Core!

[roji]

dotnet/efcore#7269 was only blocking for scaffolding support, which isn't very critical. If I remember correctly I already added support for built-in range types (e.g. int4range, daterange...) to the current dev branch, so if you're feeling brave you can try to compile the provider yourself and use ranges (would love to hear about it).

Non-built-in ranges (which you can create with CREATE TYPE) aren't supported yet and would require a more significant refactor in how type mapping works.