Accept optional strings as number arguments

[ptodev]

PR Description

Currently, if you try to assign a non-secret optional secret to a number argument, you'll get an error should be number, got capsule. For example, this generates an error:

local.file "test" {
  filename  = "/Users/paulintodev/Desktop/test_input.txt"
  is_secret = false
}

prometheus.remote_write "mimir" {
  endpoint {
    ...
    queue_config {
        max_samples_per_send = local.file.samples.content
    }
  }
}

On the other hand, this wouldn't generate an error:

prometheus.remote_write "mimir" {
  endpoint {
    ...
    queue_config {
        max_samples_per_send = "1000"
    }
  }
}

The behaviour of a non-secret optional secret and that of a string needs to be consistent, so this PR will fix it.

PR Checklist

• CHANGELOG.md updated
• Documentation added
• Tests updated
• Config converters updated