[ReRanker] Add reranker-delimiter, adapt it slightly to work with SeeKeR

[jxmsML]

Patch description
Changes:

1. Add reranker_delimiter arg as otherwise it will share the same delimiter as the transformer generator agent
2. Add field to change the inference opt key. For SeeKeR, the inference key in op is 'drm_inference' not 'inference'
3. Add the get_observations_for_reranker function. Again useful for SeeKeR case, where the observations come from observations['knowledge_agent']

Testing steps

Other information