Support provider network mirror with pull-through cache

[oliviermichaelis]

I thought about adding support for the Provider Network Mirror Protocol to the project. In addition, there would be an opportunity to implement a pull-through cache for requested providers into the boring-registry.
Thus not only proxying the network traffic, but also maintaining a (local) mirror of providers from upstream registries. The goal would be to increase the level of autonomy e.g. in case an upstream registry is unavailable. Another benefit could be a speedup in terraform runs in certain circumstances because of network locality.

The Terraform CLI would need to be configured accordingly to use the Provider Network Mirror Protocol:

provider_installation {
  network_mirror {
    url = "https://example.com:5601/v1/mirror/"
  }
}

I started working on the feature and built a PoC to see if it's feasible (and sensible at all) to implement. So far, feature-wise, everything seems to be working.
The code is not in an ideal shape right now, but nevertheless I'd like to post a PR to get the discussion started as early as possible to see if the whole idea and approach make sense at all :)

[oliviermichaelis]

This feature was added by #143