Add schema for backends

[radeksimko]

Current Version

v0.8.0

Use-cases

Provide schema so that LS can provide completion candidates for backend types (e.g. remote, s3, azurerm, gcs etc.) and any configuration relevant for each backend type.

Proposal

Add backend schemas to https://github.com/hashicorp/terraform-schema

[github-actions]

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