Add warning when lambda function may timeout

[imjoehaines]

Goal

Adds a notify for lambda functions when they are likely to timeout — by default this happens 1 second before the timeout occurs but can be configured with the new lambda_timeout_notify_ms option:

@bugsnag.aws_lambda_handler(lambda_timeout_notify_ms=2000)
def my_handler(event, context):
    pass

There's some complexity here because the notify happens off of the main thread so any thread-local data wouldn't be present in the event. In order to include all of the relevant information (e.g. metadata, breadcrumbs, feature flags) we need to manually copy it over into the timer thread