PostgreSQL determine `Column#type` through corresponding OID. #7814

This patch replaces the `case` in `PGColumn#simplified_type` with a call to `OID::Type#simplified_type`. A column with a given OID now actually behaves that way. This will allow to register user defined `OID::Types` without further configuration.

I ran the whole test suite and compared the old to the new types.
Following is the list of types that did change with this patch:

```
DIFFERENT TYPE FOR mood: NEW: enum, BEFORE:
DIFFERENT TYPE FOR floatrange: NEW: floatrange, BEFORE: float
```

The `floatrange` is a custom type. The old type `float` was simply a coincidence
form the name `floatrange` and our type-guessing.