refactor(c/driver/postgresql): Factor out Postgres type abstraction and test it independently of the driver

[paleolimbot]

The existing type.h/type.cc provides a limited abstraction of the postgres type system; however, eventually this driver will need to deal with range, domain, array, and record types. This PR factors out existing behaviour and includes a SetSchema() method to provide the default mapping from the postgres -> arrow type systems. This PR doesn't provide full type support but connects enough wires that future support for these types is more straightforward.

Many commits on this branch also implemented a COPY -> Arrow converter abstraction that can handle range, domain, array, and record types. I moved that to a separate branch ( paleolimbot#1 ) because it has an orthogonal scope to this one. If that's of interest I'll continue with that too (which would add support for Array, Domain, and date/time types in results).

[lidavidm]

Cool! This looks much better than the existing approach. I had a few questions but LGTM

[lidavidm]

nit/format: why not enum class and follow the convention of kUninitialized, etc.?

[paleolimbot]

Good call on the enum class!

I usually see ALL_CAPS_STYLE_VALUES in Arrow C++...is the kXXX convention more common/the style used here?

[lidavidm]

kFoo is the recommended style: https://google.github.io/styleguide/cppguide.html#Enumerator_Names

Arrow is inconsistent about it, partly because it often follows this pattern instead:

class Foo {
    enum Bar { // note: NOT enum class
        CONSTANT,
    }
};

Foo::CONSTANT;

[paleolimbot]

I see! I'll tackle the gnarly rename tomorrow.

[paleolimbot]

Ok! Enum values are fixed.

[lidavidm]

Looks like there are some MSVC specific errors + you might need to dllexport some of the new things to use them in tests

[paleolimbot]

I tried a few ways to get the tests to work on Windows but none of them worked...for now I just put everything back in the header. If you're particularly opposed to that I could try harder but it may take me a bit to learn the requisite linking-on-windows behaviour.

[lidavidm]

No worries.

[lidavidm]

Thanks!