Allow full class names (with namespaces) for Action Mailbox routes

Currently, when declaring mailbox routes, it’s only possible to pass a string/symbol, which gets camelized into a class name:

`routing /^save@/i => :forwards routing /@replies\./i => :replies```

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Would it make sens to allow the following syntax:

routing /noa/i, class_name: ‘LongHaul::NOAMailbox’ routing /delivery/i, class_name: ‘LastMile::DeliveryMailbox’

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This way, all the mailboxes wouldn’t have to live in the same directory.