feat: add option to read Gitlab Runner Registration token from SSM

[Conoreby]

Description

Adds the ability to read the Gitlab registration token from SSM. If no registration token is passed in, it will look in SSM to find the token to use. This prevents the token from being leaked as part of the user_data.

module "gitlab_runner" {
  # ...
  gitlab_runner_registration_config = {
    registration_token = "" # this is the default value too
    # ...
  }

  secure_parameter_store_gitlab_runner_registration_token_name = "name-of-ssm-parameter-holding-the-registration-token"

Closes #776
Precondition for #186 to get rid of pre-registered runners.

Migrations required

NO

Verification

I modified the runner-default example to not pass in a registration token and added the token to SSM instead. Then I started up the runner and confirmed that it successfully registered with Gitlab.

[github-actions]

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[npalm]

@Conoreby thanks, can you have a look on the failing workflows. Can you also add a note to the top-level readme about the options users have to provide the registration token.

[kayman-mk]

You made a good job here to make the module safer. Thank you.