Preload providers

[appilon]

This PR creates a generate directive to

• query the registery for the list of official providers
• generate a config file with a required_providers block for all those providers (no version constraint, so TF will assume latest)
• use terraform-exec to initialize/download all the providers
• use terraform-exec to extract the provider schemas
• use vfsgen to embed the json into a go file
• modifies the root module package to use the preloaded provider schemas until the rootmodule schemas are loaded

May depend on #301

TODO

vfsgen uses a common pattern of alike tools in working with the http.FileSystem interface. It would be nice to find an alternative that can follow the new embed proposal, which builds off the new filesystem proposal (something we have begun to adopt). Perhaps we could fork vfsgen and reimplement it to work with the proposed types.

Closes #292

[radeksimko]

vfsgen uses a common pattern of alike tools in working with the http.FileSystem interface. It would be nice to find an alternative that can follow the new embed proposal, which builds off the new filesystem proposal (something we have begun to adopt). Perhaps we could fork vfsgen and reimplement it to work with the proposed types.

I would hope/expect that if vfsgen is still an actively maintained library, they'll have interest in making it compatible with the native interfaces. Either way though I wouldn't worry about this too much until the proposal is actually implemented and available in a stable Go version.

[radeksimko]

I left you some comments inline, the most important one being about the panic.

From practical stand point I would probably only stick to the official providers for now as the generated file already has 3.5M. I think it's a reasonable cost to pay for better UX 👍 , but we have to keep in mind that maintaining bigger files in the repo can negatively affect the longer-term developer experience (nobody enjoys cloning large repositories) and potentially also end-user UX (nobody enjoys downloading large binaries).

[radeksimko]

I apologise ahead for the conflicts, but I'd recommend merging #307 first and rebasing this PR.

[radeksimko]

LGTM

🚢

[ghost]

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