added DialWithConnection function

[sz-po]

In case of connecting through a bastion host, it is necessary to allow dial using existing net.Conn.

Look: https://stackoverflow.com/questions/35906991/go-x-crypto-ssh-how-to-establish-ssh-connection-to-private-instance-over-a-ba

[helloyi]

I think it would be more reasonable to add a Client.Dial(network, addr string, config *ssh.ClientConfig) (*Client, error).

The reasons are as follows:

1. Consistent with the semantics of the ssh package, the difference is that the sshclient package does a top-level encapsulation and returns the Client object.
2. Client.DialWithPasswd(addr, user, passwd string) (*Client, error) and other Dial* methods can be extended more flexibly and consistently.

Based on the above, to ensure the simplicity of sshclient.

[helloyi]

Client.Dial(network, addr string, config *ssh.ClientConfig) (*Client, error) has been added.

[Aaron-cdx]

hi @helloyi for this PR, I think it should be merged because for current code usage can't support connecting to the socks5 proxy and then connecting to the server.

client0, err := Dial("tcp", "host:port", config{})
if err != nil {
  handleErr(err)
}
defer client0.Close()

client, err := client0.Dail("tcp", "host:port", config{})
if err != nil {
  handleErr(err)
}
defer client.Close()

but if merge this PR, we can pass the conn which is dialed by socks5 proxy, which can help us connect the server successfully.
Can you help me if you have time? Because I am using your library now and our proxy is socks5. Thanks!

[helloyi]

I feel the power of DialWithConnection!
Thanks @sz-po !

[helloyi]

@Aaron-cdx
Would you provide an example of how you use DialWithConnection?
Please add usage examples to README.
Thanks!

[Aaron-cdx]

hi @helloyi Okay, I'll submit a PR later for this.