Adapt spin new so "decentralized templates" can be provisioned

[jaydrogers]

👉 Describe the problem

• All templates for Spin must be committed to the repository, meaning it's difficult to scale from a community perspective
• People want to contribute templates, but they don't know how

👥 Problem evidence & reach

YouTube comment (https://www.youtube.com/watch?v=I_dq-kRDztI):

🥰 Describe the "impact" on users?

This would make the community be more self-sustained and improve compatibility for other people to start using Spin.

🏆 How to solve this problem

Change the spin new command to read from other repositories

We can change spin new to read from external GitHub repositories. Think of this working like Docker Hub. There can be two types of repositories:

1. Official Templates
2. Community templates

Official template example

spin new laravel

That could be under a GitHub repo of spin/laravel (or something like that).

If someone wanted to roll their own template, it would just be:

spin new myexample/repo

This would pull from that repository and initialize the project with the files of their choice.

Guides & Documentation

• Create a developer guide on how to build and test their own templates

[jaydrogers]

This is being moved into development 🥳

Future updates are here: #74