InMemoryBroker startup method does not call result_backend's startup method

[matiaskotlik]

The InMemoryBroker bundled in taskiq does not call startup() on its result_backend.

The default result_backend for InMemoryBroker (InmemoryResultBackend) does not implement startup() so if you do not override the result_backend with with_result_backend(), this bug has no effect.

Using any other result backend (that implements startup()) with the InMemoryBroker will have unexpected results since startup() will never get called on the result backend.

It's a one line fix. Great first PR for any beginner. I will push a fix later this week if no one beats me to it.

[s3rius]

Please do it. Any contribution is valuable, thanks.