Serializable AddressGeolocation

Hi,

The AddressPicker form field is great, and will return a ``AddressGeolocation`` object.

This PR proposes a small improvement, where this value object is serializable.
It can be useful for example if you want to directly map it to a Doctrine `object` type. No BC breaks.

I would have liked to add some tests, but it seems at the moment composer dependencies can not be resolved?