Add client demo that implement open direct tcpip.

[handewo]

This demo works like ssh -L <bind_addr:forward_addr> -N <host>
Known issue: forward data sometimes incorrect since not handle ChannelMsg::WindowAdjusted, but I think it's not important as demo.

[Eugeny]

Thank you for the PR. I'd rather not overload the new example with certificate handling. If you'd like, you can instead add the certificates support to client_exec_simple

[handewo]

Thank you for the PR. I'd rather not overload the new example with certificate handling. If you'd like, you can instead add the certificates support to client_exec_simple

No problem. I've added.

[Eugeny]

Could you please also remove the usage of async_trait to fix the build? It's now optional in the crate and shouldn't be used in the examples

[handewo]

Could you please also remove the usage of async_trait to fix the build? It's now optional in the crate and shouldn't be used in the examples

Done.