[C++][Parquet] Use C++17's nested namespaces

### Describe the enhancement requested

Part of https://github.com/apache/arrow/issues/36620

Before:

```
namespace parquet {
namespace arrow {
```

After:

```
namespace parquet::arrow {
```

I guess most of these is under `parquet/arrow` directory.

### Component(s)

C++, Parquet