Support list expansion based on dynamic variables

[ChrsMark]

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:

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.

[ph]

Interesting, what you are asking is actually similar to yaml reference using the ampersand.

[ChrsMark]

Yeap but we need to handle this on Agent's code since on pure yaml it is not supported to merge lists/arrays dynamically: yaml/yaml#35

[ph]

I believe we would need to change quite a fews thing internally to support object like structure. Maybe we should even make this an explicit decision with a prefix key keyword. Maybe a bad example syntax but to better illustrate what I mean, ${local_dynamic.proces:expand} or ${local_dynamic.proces:object}