[Feature request] Add parameter to vanilla-specs to support locust --stop-timeout

[aleks-vodolazko]

Is your feature request related to a problem? Please describe.
Sometimes some conditions/operations should be executed before launching the on_stop method when timeout is reached.
Example:

        with client.request(your_args, catch_response=True) as resp:
            if resp.status_code == 200:
                self.foo += 1    // This action will be ignored if run-time timeout is fired before the response is received. 
                resp.success()
            else:
                resp.failure()

Describe the solution you'd like
This type of issue can be handled by adding a special timeout to finish the current task execution.
Locust has a special flag to set -s STOP_TIMEOUT, --stop-timeout. It could be 0 by default, but it would be great to have it configurable.

Describe alternatives you've considered
If we can have an additional field in .yaml file such as "customParameters" or "customFlags" or "additionalFlags" that will be added at the end of the locust launching command - it would provide full support of all locust flags.

Additional context
locustio/locust#1099

[AbdelrhmanHamouda]

This is a valid use case and would be a welcomed addition to the project