Add support for field name based composite type scanning

[WhiskeyJack96]

This is pretty rough, but before going through the process to polish it and test it I wanted to get something up to see if this would be accepted (or if theres a "better" way)

[jackc]

The fundamental difficulty with scanning by names in composite types is that composite field names are not included over the wire. They are sent positionally. So I don't think it is possible for row(...) or subquery results. I guess it might be possible for specific registered types to use the OID to look up the previously determined field names. I'm guessing that's the approach you are taking? I looked at the code but I didn't quite follow it.

I suppose this might be a reasonable approach to solve the problem. But I actually wonder if the problem needs to be solved. Composites are always returned in a specifically defined order. I just use a struct defined where the position of the fields matches the position of the fields on the PostgreSQL type. I haven't encountered the need for anything past that.