Parser: pass configuration object

[kwinkel]

Is your feature request related to a problem? Please describe.
When using not included plugins it is not a great experience using existing fields of the Flow/ClusterFlow CRD to configure any custom plugin.

Describe the solution you'd like
Please provide a CRD field for parsers to allow a configuration like this:

spec:
  filters:
    - parser:
        parse:
          type: myplugin
          configuration: |
            myconfkey somevalue,someothervalue

Describe alternatives you've considered
Currently I'm using the expression field which limits custom plugins to use the CRD. This workaround is very limited.