3rd party extensions

[carletex]

Allow people to install 3rd party extensions/plugins ** from GitHub URLS

e.g.

npx create-eth@latest --extension=https://github.com/buidlGuidl/eip-712-extension
# even branches (with ":"? after the URL)
npx create-eth@latest --extension=https://github.com/buidlGuidl/se2-extensions:eip-712

We'd have to define a set of rules for people to create these extensions, e.g.:

• extension.ts. Some metadata + CLI options
• packages / folders / files / dependencies: as we do with our custom core extensions (https://github.com/scaffold-eth/create-eth/blob/main/contributors/RFC-extensions.md)

Also a good way to develop these extensions (--dev mode, etc)