Support for Go's built-in context in Dial methods

[jonathroth]

Currently ziti.Context's Dial/DialWithOptions/DialAddr does not support passing golang's built-in context.Context object for cancellation While it does support passing a ConnectTimeout, I'd like to be able to cancel the Dial attempt on-demand and not wait for a timeout

[plorenz]

I think this is a good idea and am putting it onto the backlog.