Allow iterable syntax for Transformer list

## Description

This should allow to chain transformers of the same type without the `#num` suffix hack.
We should also deprecate the old way.

Example :
```yaml
transformers:
    - code: mapping
      options:
          keep_input: <...>
          mapping: <...>
    - code: mapping
      options:
          keep_input: <...>
          mapping: <...>
```