Support for sub_inputs in extra plugins

[lisuml]

Description of PR There are cases when a sub inputs have to be configured within same file as the main input, for example in cloudwatch plugin, where you may need to also add [[inputs.cloudwatch.metrics]] and [[inputs.cloudwatch.metrics.dimensions]] along with the main [[inputs.cloudwatch]] input.

Type of change Feature Pull Request

[dj-wasabi]

Thank you!