Better sub inputs handling (#193)

* Support for sub_inputs in extra plugins

* fix: properly handle lists as TOML arrays

- Updated the Jinja template to detect and serialize lists using |to_json,
  ensuring Telegraf interprets them correctly (e.g. names = ["item1","item2"]).
- Removed fallback logic for single items, so we consistently treat all sub_input
  values as lists of dictionaries.

---------

Co-authored-by: Michał Lisowski <mlisowski@sentient.ie>

README.md

@@ -342,13 +342,13 @@ sqs:
     - statistic_include = ["average"]
   sub_inputs:
     metrics:
-      - names = [
+      - names: [
           "ApproximateAgeOfOldestMessage",
           "ApproximateNumberOfMessagesVisible",
         ]
     metrics.dimensions:
-      - name = "QueueName"
-      - value = "*"
+      - name: "QueueName"
+        value: "*"
 ```
 
 ## Dependencies