decoder: validateBody should support schema extensions

[dbanck]

Requires #287

Problem Statement

In the work on functions & expressions we introduced the concept of extensions for handling count and for_each. Both aren't part of the regular attributes for a block, but are introduced by enabling the specific extensions for a block.

If the extensions are enabled for a block, we should not raise diagnostics for those attributes. If not, it's helpful to raise them.

Proposal

• Update validateBody to include BodyExtensions when validating a file
• When encountering an unknown attribute see if it's an attribute belonging to an extension AND the extension is enabled for that block
• Support the following extensions
  • count
  • for_each
  • dynamic might just work™
• Add test cases for each extension

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Part of hashicorp/vscode-terraform#720