Add support for disabling I/O over TUN/TAP support

[kanpov]

In my case, I only need the builder from tokio-tun to create the TUN/TAP device and don't want any of the I/O over it, but the library currently requires that. By gating this behind a feature or adding a feature to disable it, the tokio net dependency wouldn't need to be pulled in for these cases.

[kanpov]

Or, make a base crate like core-tun that would do the TUN/TAP creation and have tokio-tun depend on it with the async I/O added.