feat: add autoscaler runner instance ready command option (#1190)

## Description

This adds the optional instance_ready_command parameter for the
docker_autoscaler.

This feature was added in Gitlab 16.11. See gitlab documentation for the
[runners.autoscaler]:
https://docs.gitlab.com/runner/configuration/advanced-configuration.html#the-runnersautoscaler-section.

Example value:
`instance_ready_command = "cloud-init status --wait"`

## Migrations required

 No

## Verification

Updated the runner_worker_docker_autoscaler by including the
instance_ready_command

```
runner_worker_docker_autoscaler = {
    fleeting_plugin_version = "1.0.0"
    connector_config_user = "ec2-user"
    key_pair_name = "runner-worker-key"
    max_use_count = 100
    update_interval = "1m"
    update_interval_when_expecting = "2s"
    capacity_per_instance = var.job_capacity_per_instance
    instance_ready_command = "\"cloud-init status --wait || test $? -eq 2\""
  }
```

Confirmed this gets added correctly to the config.toml by enabling the
var.debug_write_runner_config_to_file.

Result on the local_file.config_toml[0]:
```
...
[runners.autoscaler]
      plugin = "fleeting-plugin-aws"
  
      capacity_per_instance = 1
      update_interval = "1m"
      update_interval_when_expecting = "2s"
  
      max_use_count = 100
      max_instances = 1
  
      instance_ready_command = "cloud-init status --wait || test $? -eq 2"
 ....
```

If no value is given for the instance_ready_command the config.toml will
default to
```
...
[runners.autoscaler]
    plugin = "fleeting-plugin-aws"

    capacity_per_instance = 1
    update_interval = "1m"
    update_interval_when_expecting = "2s"

    max_use_count = 100
    max_instances = 1
    
    instance_ready_command=""
 ...
```

Author: maxvandermeij

main.tf

@@ -131,6 +131,8 @@ locals {
       runners_update_interval                = var.runner_worker_docker_autoscaler.update_interval
       runners_update_interval_when_expecting = var.runner_worker_docker_autoscaler.update_interval_when_expecting
 
+      runners_instance_ready_command = var.runner_worker_docker_autoscaler.instance_ready_command
+
       runners_autoscaling = [for config in var.runner_worker_docker_autoscaler_autoscaling_options : {
         for key, value in config :
         # Convert key from snake_case to PascalCase which is the casing for this section.

template/runner-docker-autoscaler-config.tftpl

@@ -9,6 +9,8 @@
     max_use_count = ${runners_max_use_count}
     max_instances = ${runners_max_instances}
 
+    instance_ready_command="${runners_instance_ready_command}"
+
     [runners.autoscaler.plugin_config] # plugin specific configuration (see plugin documentation)
       name = "${docker_autoscaling_name}"     # AWS Autoscaling Group name
 

variables.tf

@@ -610,6 +610,7 @@ variable "runner_worker_docker_autoscaler" {
     max_use_count = Max job number that can run on a worker
     update_interval = The interval to check with the fleeting plugin for instance updates.
     update_interval_when_expecting = The interval to check with the fleeting plugin for instance updates when expecting a state change.
+    instance_ready_command = Executes this command on each instance provisioned by the autoscaler to ensure that it is ready for use. A failure results in the instance being removed.
   EOT
   type = object({
     fleeting_plugin_version        = optional(string, "1.0.0")
@@ -619,6 +620,7 @@ variable "runner_worker_docker_autoscaler" {
     max_use_count                  = optional(number, 100)
     update_interval                = optional(string, "1m")
     update_interval_when_expecting = optional(string, "2s")
+    instance_ready_command         = optional(string, "")
   })
   default = {}
 }