Add the ability to pass a search string when instantiating the photo picker

[ocollet]

Some use cases can require to show photos around a specific search, and not allow users to search themselves.

We can add a query: String? parameter to UnsplashPhotoPickerConfiguration.

• If the value is nil, we keep the current behaviour.
• If the value contains a string, the photo picker performs the search right away, and the search bar is not visible.