gmpx-place-picker styling

**Is your feature request related to a problem? Please describe.**
It doesn't seem possible to style gmpx-place-picker except with a few css variables. What if I want to change borders, remove rounded corners.. ?

**Describe the solution you'd like**
The closed shadow dom prevent any interference. Either expose more css variables to be more flexible, or don't close the shadow dom

**Describe alternatives you've considered**
None

**Additional context**
None