Add exception.cause?.stack to the `ATTR_EXCEPTION_STACKTRACE` attribute

[iblancasa]

I think it would be worth looking at the discussions that led to the exception.* attributes in the spec/semconv to see if cause and/or code (or other exception attributes) were discussed.

I think it would be worthwhile to have a follow-up issue to consider adding exception.cause?.stack to the ATTR_EXCEPTION_STACKTRACE attribute.

Originally posted by @trentm in #6385 (comment)