Support list expansion based on dynamic variables

**Describe the enhancement:**
As a user I would like to have the option for expanding a `yml` list using a list coming from a dynamic provider.

**Describe a specific use case for the enhancement or feature:**
In case I want to expand the list of processors with processors coming from a dynamic provider, I'm not able to achieve this.

Example:

```yml
providers:
local_dynamic:
items:
- vars:
key: value1
procs:
- add_fields:
fields:
custom: bar
target: dynamic
- add_fields:
fields:
hints: true
target: kubernetes

inputs:
- type: logfile
enabled: true
streams:
- paths:
- /var/log/${local_dynamic.key}
processors:
- add_fields:
fields:
custom2: foo
target: baz
${local_dynamic.procs}
```

In the above example I would like to expand the `processors` list with the list coming from the local provider.