Add color customization support

[markmur]

What are you trying to accomplish?

Example configuration for light, dark and web options:

ShopifyCheckoutKit.configure({
  colorScheme: ColorScheme.web,
  colors: {
    ios: {
      backgroundColor: '#f0f0e8',
      spinnerColor: '#2d2a38',
    },
    android: {
      backgroundColor: '#f0f0e8',
      spinnerColor: '#2d2a38',
      headerBackgroundColor: '#f0f0e8',
      headerTextColor: '#2d2a38',
    },
  },
});

Example config for automatic:

ShopifyCheckoutKit.configure({
  colorScheme: ColorScheme.automatic,
  colors: {
    ios: {
      backgroundColor: '#f0f0e8',
      spinnerColor: '#2d2a38',
    },
    android: {
      light: {
        backgroundColor: '#f0f0e8',
        spinnerColor: '#2d2a38',
        headerBackgroundColor: '#f0f0e8',
        headerTextColor: '#2d2a38',
      },
      dark: {
        backgroundColor: '#f0f0e8',
        spinnerColor: '#2d2a38',
        headerBackgroundColor: '#f0f0e8',
        headerTextColor: '#2d2a38',
      },
    },
  },
});

[cianbuckley]

If you create a type for the config (eg a record class might do), all these map checks would go away and clean up the code. You could do all the config validation in this function

[cianbuckley]

Is it possible to use an external lib like jackson? I think you could convert a map direct to a POJO. I reckon it might clean up the code a bit.

[markmur]

@cianbuckley and I paired on this to improve the java codebase, but we decided to handle it in a follow up to avoid blocking progress here

[cianbuckley]

LGTM