decoder: validateBody should support AnyAttribute

[dbanck]

Requires #287

Problem Statement

Some blocks don't have a static attribute schema, where we can compare each attribute in the configuration to an attribute in the schema. Blocks with an AnyAttribute schema allow any key as an attribute, while only the value has to follow specific rules.

Proposal

• Update validateBody to validate blocks with AnyAttribute correctly
  • Might be just skipping validation for those
• Blocks in the core schema:
  • locals
  • required_providers within terraform
• Add test cases with AnyAttribue

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Part of hashicorp/vscode-terraform#720