ENH: Set default storage for strings globaly
#### Is your feature request related to a problem?

As far as I know, the only way to use the new string dtypes, is to set them manually either via the param `dtype` if it's supported or using `astype` otherwise the string will be stored as `object`.

#### Describe the solution you'd like

I'm aware that the string dtype is experimental, but it would be easy for us to play with it in large projects if there's a way to change the default behavior globally which can also be used in the future to set the storage for `StringDtype`. 
