feat: add ExternalImageRepository

[kkafar]

Description

This PR aims to add External Image Repository native mechanism, where anybody on the native side can integrate with react-native-screens and provide us with an image
during application runtime, so that int can be e.g. used for tab bar item icon.

This will allow for integration with third-party image solutions, e.g. expo-image.

What we need to do:

1. Expose the image repository on both main platforms in a way, that it is possible to access their instances from outside the package.
2. Integrate our image loading code with the image repository.
3. Unify image specification across all of our components & introduce shared utilities to centralise image loading / handling flows.

Changes

Before & after - visual documentation

Test plan

Checklist

• Included code example that can be used to test this change.
• For visual changes, included screenshots / GIFs / recordings documenting the change.
• For API changes, updated relevant public types.
• Ensured that CI passes