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ARROW-13346: [C++] Remove compile time parsing from EnumType

Requires a slightly more verbose declaration:

```c++
struct Color : EnumType<Color> {
  using EnumType::EnumType;
  static constexpr EnumStrings<3> values() { return {"red", "green", "blue"}; }
  static constexpr const char* name() { return "Color"; }
};
```

but doesn't do any string processing at compile time beyond computing the length of each string.

Closes #10726 from bkietz/13346-Remove-compile-time-parsi

Authored-by: Benjamin Kietzman <bengilgit@gmail.com>
Signed-off-by: Benjamin Kietzman <bengilgit@gmail.com>