Support ayon-core as main addon of startup logic

[iLLiCiTiT]

Changelog Description

Bootstrap logic can use ayon_core addon over openpype as main cli handler.

Additional info

This is forwards compatibility for ayon-core addon that will be released soon. The main difference is that ayon core will have main function that is called instead of directly working with click main command. That gives option to pass some of initialization steps from ayon-launcher to ayon-core addon, like change of PYTHONPATH and set-up global environments.

The changes should be backwards compatible with openpype addon.

Testing notes:

1. Use ayon-core in your bundle.
2. Launch ayon-launcher with the bundle.
3. All should work fine.