Need a way to subscribe to DriverWarning as events with a callback

With warnings.warn, if users have to asynchronously process driver warnings, then they need to override the warnings.showwarning function with a custom handler which would process driver warnings differently. This implies a global override, more things for users to maintain and a possibility of not handling certain other warnings in the application correctly. Would be good to have a warning event mechanism built into driver session which the users can subscribe to, by passing a callback function. This event mechanism can co-exist with the current warnings.warn. This way, users can adopt whatever approach makes sense for the application.

ie:

• if they want to raise exceptions for warnings, then they can simply do
  warnings.simplefilter("error", DriverWarning)
• if they want to asynchronously handle warnings without stopping the execution flow, then they can do
  session.warning_event.subscribe(driverwarninghandler(DriverWarning)) and define

def driverwarninghandler(DriverWarning):
    <log to file, set a flag, push to a queue, raise few warnings and ignore others, etc>

Start by reviewing the driver session's existing DriverWarning and warnings.warn handling; the issue does not name specific files or tests. Define the callback subscription behavior and verify that asynchronous handling can coexist with the current warning mechanism.