decoder: validateBody should ignore missing provider schemas

[dbanck]

Requires #287

Problem Statement

We currently compare all blocks and attributes against the available schema. We don't distinguish between a missing schema or a block that doesn't accept any attributes. Since we only bundle the ~250 official and partner providers with terraform-ls, it is likely that we will encounter providers for which we don't have a schema yet.

Instead of raising diagnostics for unknown attributes / blocks we should not raise any.

Proposal

• Update validateBody to skip resources/blocks when validating a file if we don't have a schema
  • Return an additional boolean from mergeBlockBodySchemas to indicate if merging was successfull
  • If merging is successful we can continue as normal
  • If not, we have to skip some validations, like required attribute/block detection
  • If we encounter a attribute/block that still exists in the schema (must be part of the core schema then), we can return to normal validation for that one

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Part of hashicorp/vscode-terraform#720