Supply backend config as vars a "rawFlags" k/v pairs similar to CLI

See https://github.com/hashicorp/terraform/blob/d2e999ba1f2a4a12f29e65f4ede95882f55e15f0/command/meta_config.go#L387-L392

Start by reading Terraform's command/meta_config.go at lines 387-392, linked in the issue, to understand how the CLI represents backend configuration. Trace the corresponding terraform-exec backend configuration entry point and define what exposing equivalent rawFlags key/value pairs would require; done means callers can supply backend config in that form.

Written by the indexing model from the issue text.