feat: capture caused_by and return it as part of BulkIndexerResponseItem

[kyungeunni]

closes #166

Parse caused_by from the response, and add it to BulkIndexerResponseItem so that it can be logged.

TODO

• Decide whether to make it optional (verbose:on/off) : having it by default sounds reasonable as it's an important bit of information

[axw]

Decide whether to make it optional (verbose:on/off)

This bit is critical. Do you have a plan for how we are going decide this?
I think the PR should probably be in draft until this is decided.

[kyungeunni]

@axw Having discussed this with the team, it seems it'd make sense to log this by default as it would be helpful to understand the error better even though the logs become a bit verbose. Do you have any concern about this?

[axw]

I'm not concerned about verbosity. My only concern is whether we might leak document details in the caused_by. Do we know that that won't happen?

[kyungeunni]

The decision on this was to make it work similar to #165 (log it when opting in) as we are not 100% certain about caused_by not leaking any document details. Closing this for now, and will revisit it when we are happy to go ahead with #165.