Extract sub-exceptions from Exception Groups

[imjoehaines]

Goal

Python 3.11 added Exception Groups (see PEP 654) as a way of aggregating several unrelated exceptions into a single Exception-compatible object

bugsnag-python currently supports reporting Exception Groups, but there's no visibility of the sub-exceptions contained within the group. This is problematic as often the group won't contain enough information to debug the problems that caused it — the contained exceptions should be unrelated, so it's difficult to go from the group → its sub-exceptions

This PR extracts the sub-exceptions from Exception Groups, so that they are reported to bugsnag along with the group. This is very similar to how we report AggregateExceptions in the .NET notifier

Design

In order to avoid truly gigantic lists of errors in the dashboard, we don't recurse into sub-exception groups or their cause/context. The error reporting API also doesn't support tree-shaped hierarchies of errors, so this structure would be difficult to deal with in bugsnag anyway

For example, this exception group:

will be reported as:

This should still contain enough information to debug the errors and, if not, the exceptions can be reported separately by catching the ExceptionGroup itself

[imjoehaines]

Our fully_qualified_class_name is appending builtins. here, which is not technically wrong but is unexpected

Fixing this is trivial but I've split this into a separate task as:

1. I'm not sure why this happens for ExceptionGroup but not other built-in exceptions
2. fully_qualified_class_name is also used when applying ignore_classes, so we need to be careful not to change its behaviour in a way that would break ignoring errors

[imjoehaines]

This type: ignore comment is because we run mypy on Python 3.5 (as well as 3.11) where it has no knowledge of BaseExceptionGroup, so complains about using exception.exceptions