Make delete action on content_modifier processor accept list of keys

[anderssynstad]

Is your feature request related to a problem? Please describe.
No.

Describe the solution you'd like

processors:
  logs:
    - name: content_modifier
      action: delete
      key:
        - foo
        - bar
        - baz

Describe alternatives you've considered
N/A

Additional context
This would make the configuration a lot less bulky if one is removing a large amount of keys.