feat: ability to customize SelectionDot component

[robinvw1]

Description

The current selection dot is 'design-wise' quite opinionated. It has a light shadow and an outer ring.

Instead of introducing a lot of new props (no offence https://github.com/margelo/react-native-graph/pull/21) to customize the look-and-feel of the selection dot, I went for a flexible way to provide your own SelectionDot component.

Demo

Default situation	With custom selection dot