Allow setting -parallelism= flag

### TL;DR

Setting `terraform plan -parallelism=100` can give a ~3x speedup for large projects. I'd like to be able to set this with `guardian plan`, but I don't see an option for it: https://github.com/abcxyz/guardian/blob/main/cli.md#plan

### Detailed design

```markdown
This could either be an additional flag for guardian plan, or some generic way of adding additional terraform flags, such as `guardian plan -terraform-flags="-parallelism=100"`.
```

### Alternatives considered

```markdown
Just being slightly more patient :)
```

### Additional information

Evidence for the speedup when using `guardian run -- ...`:


The issue is about adding a `-parallelism` flag or a generic `-terraform-flags` option to the `guardian plan` command. Start by examining the CLI command structure in the codebase, likely in a `cmd/` directory. Look for the `plan` command implementation to see how terraform is invoked. Adding a flag involves parsing it and passing it to the terraform command. Check if there are existing patterns for forwarding flags. Running the existing tests for the plan command will verify the change.

Rédigé par le modèle d'indexation à partir du texte de l'issue.