Source provider schemas directly from binaries via gRPC

[radeksimko]

Current Version

0.11.0

Context

As explained in #354 and probably in some other related issues, we need to expand ways we source provider schemas.

We currently employ two mechanisms:

• Terraform CLI (terraform providers schema -json)
• preloading JSON export

which provide some basic coverage.

We plan to also source from the Registry API, but this is dependent on Registry changes: #193

Proposal

Source schema directly from provider binaries via existing gRPC protocol, i.e. effectively call GetSchema method to improve the coverage.

This means we will also need to extract the mechanism for discovering installed plugins in .terraform, which also differs between Terraform versions, so we know where to find the binaries and how to identify them.

If it proves to be reliable enough that it can be treated as equivalent to terraform providers schema -json we could remove the CLI method entirely.