Allow OneShotHandler's max_dial_negotiate limit to be configurable.

[wngr]

This simply moves the max_dial_negotiated field of OneShotHandler into its config (and thus making it configurable). The default is set to the formerly hard-coded value of 8.

[romanb]

Looks good to me, thanks!

[romanb]

Released as libp2p-swarm-0.27.1. Though I realised afterwards that the OneShotHandlerConfig struct is not yet #[non_exhaustive], contrary to what I thought, so theoretically there may be code that needs to be updated to compile with this patch if it makes direct use of the OneShotHandlerConfig constructor, due to the new field. I don't think it is worth yanking for that reason though, as I think most code that uses the OneShotHandler either uses the default configuration or obtains it via the Default impl for subsequent customisation. Sorry for the oversight.