To allow doing things like having a single `AbortSignal` that can
be used as a timeout for incoming connection establishment, allow
passing it as an option to the `ConnectionEncrypter` `secureOutbound`
and `secureInbound` methods.

Previously we'd wrap the stream to be secured in an `AbortableSource`,
however this has some [serious performance implications](ChainSafe/js-libp2p-gossipsub#361)
and it's generally better to just use a signal to cancel an ongoing
operation instead of racing every chunk that comes out of the source.