feat: add shuttle extensions proposal

[kjuulh]

This adds a proposal (golden scenario) for what I consider a next generation package manager and tool for a company specifically made to suit Lunars needs.

I hope that shuttle extensions even if not in the form described here will allow shuttle to act as a central hub of all your developer tools, and solve the same problems that shuttle plans solved, but for other types of domains.

• shuttle plans where specifically tailored to share deployment and project layouts, to minimize code duplication, but also to move the complexity of maintaining application infrastructure to a single part of the org
• shuttle extensions are aimed at supporting a wider and more generic type of need, such removing in-effencies of manual work, lower the barrier to entry for shipping tools to your developers, or for a team to maintain their own set of tools across services, and move those complexities to the team with the most know how. But still being able to share it broadly

Rendered view: https://github.com/lunarway/shuttle/blob/feat/shuttle-extensions/docs/features/extensions.md