action: Add new local_command action to invoke a local executable with provided input

[austinvalle]

Related Issue

N/A

Description

This PR introduces a new action to the local provider, local_command, which is intended to be a generic way to run an executable on the local machine of whatever system is running Terraform. Generally this action is like an escape hatch and is built in a similar fashion to the local-exec provisioner. (although in it's current form, actions do not have all the necessary features to consider local_command to be a replacement for local-exec)

The local_command is being introduced here, rather than the external provider, because the purpose of the only data source in the external provider is to interact with executables that are purpose-built for Terraform with a documented protocol (must produce valid JSON on STDOUT, must accept a JSON object on STDIN, etc). The local_command is meant to interact with any executable on the local machine, and thus allows the users, for example, to format their own STDIN data + arguments:

action "local_command" "bash_example" {
  config {
    command   = "bash"
    arguments = ["example_script.sh", "arg1", "arg2"]
    # In the external provider, the data is automatically encoded to JSON, here
    # we encode the data using the built-in jsonencode function
    stdin = jsonencode({
      "key1" : "value1"
      "key2" : "value2"
    })
  }
}

This control means that the practitioner can interact with essentially any executable with any data format for input that can be represented in Terraform:

action "local_command" "bash_example" {
  config {
    command   = "bash"
    arguments = ["example_script.sh", "arg1", "arg2"]
    # Send YAML instead
    stdin = yamlencode({
      "key1" : "value1"
      "key2" : "value2"
    })
  }
}

---

As this is an action, as of today, the user cannot consume any of the STDOUT data produced by the command, so we output that data via a progress message:

 $ terraform apply -auto-approve

# .. Terraform CLI output truncated for example purposes

Plan: 1 to add, 0 to change, 0 to destroy. Actions: 1 to invoke.
terraform_data.test: Creating...
terraform_data.test: Creation complete after 0s [id=4b41b541-5550-590a-9949-657e91baa346]
Action started: action.local_command.bash_example (triggered by terraform_data.test)
Action action.local_command.bash_example (triggered by terraform_data.test): 

# <STDOUT from the executable will display here once the command has finished execution>

Action complete: action.local_command.bash_example (triggered by terraform_data.test)

Apply complete! Resources: 1 added, 0 changed, 0 destroyed.

Rollback Plan

• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

No

[SBGoods]

LGTM!