feat: allow adding extra outputs

[davidspek]

Inspired by #608, this PR adds the ability to specify extra outputs both for the batch processor receiver, as well as at the last stage for metrics and logs after all processing has been done. The use case for this is for a simple way to duplicate metrics, logs and traces across multiple backends. A more complex implementation could be to have the externalServices.* values be an array, but that would make the chart much more complicated. Instead, users can use the extraConfig value to configure any additional outputs that are then referenced in these new settings.

Example:

# values.yaml
receivers:
  processors:
    batch:
      output:
        metrics: [otelcol.exporter.kafka.kafka.input]
        logs: [otelcol.exporter.kafka.kafka.input]
        traces: [otelcol.exporter.kafka.kafka.input]
externalServices:
  prometheus:
    extraOutputs: [otelcol.exporter.otlp.duplicate.input]
  loki:
    extraOutputs: [otelcol.exporter.otlp.duplicate.input]

[defanator]

What's the reason behind removing this one here?