Support more registries

[victorb]

Currently, Open-Registry acts like a npm proxy and exposes the archive we're currently building of that proxy.

However, npm is not the only package registry run by for-profit entities. Some others on the top of my mind are Packagist (composer/PHP) and Docker Hub. These registries should also, similar to the npm registry, be developed for and by the community, rather than being controlled by centralized organizations.

Currently, the architecture of the application (basically, just a proxy that caches packages in IPFS) makes it very easy to develop compatability with new packages. I'd like to rip out the parts which does the heavy-lifiting into it's own library, then we can add proxies as needed.

The order of implementing these would be the following:

• Extract out proxy-caching logic into it's own lib
• Compatible with Packagist
• Compatible with Docker Hub

Is there other registries run by for-profit companies that you think Open-Registry should provide an alternative to? Is the idea of having Open-Registry being a general package registry spreading our work to thin?