Support tfstate lookup in `ecspresso.yml`

[h-matsuo]

It would be more convenient if ecspresso could support looking up tfstate values directly in ecspresso.yml 👀

A common use case for this is when the ECS cluster name is managed by Terraform.

Currently, using the following configuration causes an error:

region: ap-northeast-1
cluster: '{{ tfstate `output.ecs_cluster_name` }}'
service: sample
service_definition: ecs-service-def.jsonnet
task_definition: ecs-task-def.jsonnet
timeout: 10m0s
plugins:
  - name: tfstate
    config:
      url: s3://sample-bucket/terraform.tfstate

Error output:

$ ecspresso diff
2025-06-15T17:51:09.262+09:00 [INFO] ecspresso version: v2.5.0
2025-06-15T17:51:09.262+09:00 [ERROR] FAILED. failed to load config file .ecspresso.yml: config parse by template failed: template: conf:2: function "tfstate" not defined

[fujiwara]

@h-matsuo Hi!

This is by design and specification in the current architecture.

The tfstate function is only available after the configuration is read because the state URLs are in the configuration file.