Add ability to change the protocol prefix string

I realize that this is a very niche feature, but I have a use case where it is required so perhaps others might as well. For context, I have an application that runs a modified Dropbear SSH server that follows the SSH protocol, but changes the protocol identification string to something else. I'm using SSHJ to communicate with both this server and standard SSH servers based on some configuration, so I needed a way to switch the protocol string without including multiple libraries.

Feel free to decline this and I'll just leave this change in my fork.