Write with custom indexName and docId not present in document itself

ElasticIO connector write method is taking currently as input a "PCollection" element ; string value will be passed as ElasticSearch document body. With methods withIdFn, and withIndexFn, we can overload Elastic document id and index name at runtime.

There is some kind of situation where indexName and/or docId are not present in document itself (for example from Google Pubsub metadata or other).

Is there a way to implement this usecase with current ElasticIO ? Otherwise, if it makes sense for you, what kind of improvment can be performed on connector ?

Imported from Jira [BEAM-11276](https://issues.apache.org/jira/browse/BEAM-11276). Original Jira may contain additional context.
Reported by: lionelsetan.