Fix extras consumers option configuration type

[mRoca]

This PR allows to use an array as extras option value for a middleware. For the moment, only scalars values are allowed, but the doc example contains an array for the swarrot.processor.signal_handler processor.

swarrot:
    consumers:
        my_consumer:
            processor: my_consumer.processor.service
            middleware_stack:
                 - configurator: swarrot.processor.signal_handler
                   extras:
                       signal_handler_signals: # Not ok for now
                           - SIGTERM
                           - SIGINT
                           - SIGQUIT
                 - configurator: swarrot.processor.max_messages
                   extras:
                       max_messages: 100 # Already ok

[olaurendeau]

Look good to me :) thanks @mRoca !