add secretsmanager plugin

[fujiwara]

provides secretsmanager_arn template function.

plugins:
  - name: secretsmanager

  "secrets": [
    {
      "name": "FOO_BAR_FROM_SECRETSMANAGER",
      "valueFrom": "{{ secretsmanager_arn `foo/bar` }}"
    }
  ]

secretsmanager_arn returns the Arn of the secret.