[Feature Request] Support changing the locale in a initiated Faker instance.

If I wanted to access the italian locale instead of the english US locale, I would have to re-initiate or create a new instance of Faker by passing `it_IT` as the locale parameter.

```python
f = Faker('en_US')
print(f.name())

f = Faker('it_IT')
print(f.name())
```

But why not we just do:

```python
f = Faker('en_US')
print(f.name())
f.set_locale('it_IT)
print(f.name())
```

It's just convenient I believe.

No file or test is named in the issue. Start by locating the Faker instance API that handles the constructor locale and checking how locale state is stored. Add a supported way to change an existing instance from en_US to it_IT, then verify that subsequent provider output uses the selected locale and that existing initialization behavior remains intact.

Written by the indexing model from the issue text.