CBFB: Add annotation support for Elastic and Splunk outputs

Similar to the recent support for Loki configuration simplification with annotations/labels - add equivalent capability for the targets requested by Anil Kumar:

• Elastic: https://docs.fluentbit.io/manual/pipeline/outputs/elasticsearch 
• Splunk: https://docs.fluentbit.io/manual/pipeline/outputs/splunk 

This may take a little more work as they're slightly more involved, particularly with the split in Elasticsearch & Opensearch. It makes more sense to target Opensearch for this.

The defaults should be simple, users can still do complex configuration via a custom Secret so we likely want to support a single index for ES and just what needs doing to get that out.

Tutorial updates as well to demonstrate this should be provided.

We should also have test stacks provided ideally, similar to the Loki one, in the OSS repo.