Summary

• Added stdin support to the terragrunt stack run command, enabling execution of console-like operations that require stdin input
• Follows the same pattern as the existing terraform console implementation

Test plan

✅ Updated test suite to include terragrunt console test with stdin
✅ Existing terragrunt tests continue to pass
✅ Example usage demonstrates how to get computed local variable values

Example usage

This PR enables users to get computed local variable values in Terratest, similar to how it's done with echo 'local.mylocal' | terragrunt run -- console:

terragruntOptions := &terragrunt.Options{
    TerragruntDir: "../examples/terragrunt-example",
    TerragruntBinary: "terragrunt",
    Stdin: strings.NewReader("local.mylocal"),
    ExtraArgs: []string{"console"},
}

// Run terragrunt stack run -- console with stdin
output, err := terragrunt.TgStackRunE(t, terragruntOptions)

// Output will contain the computed value of local.mylocal
assert.Contains(t, output, "expected_value")

This addresses the client request for using Terratest to get computed values of local variables, providing the same functionality as the CLI command echo 'local.mylocal' | terragrunt run -- console.