Support for client-go credential plugins

[tintoy]

Docs here.

See felixfbecker/PSKubectl#54 for further details.

[tintoy]

Ugh - this is ugly; the old-style credential commands took a list of arguments but the new client-go credential plugins (i.e. "exec") take a simple string containing all arguments (presumably in platform-specific format).

This means we need to support BOTH a list of arguments AND a simple string containing all arguments (and propagate this all the way through to System.Diagnostics.Process) because platform-specific quoting and escaping of command arguments is a bit of a nightmare).

[felixfbecker]

Here's an idea: Could you deserialize the property directly into a System.Diagnostics.ProcessStartInfo? It has both ArgumentList and Arguments (string), which you could set respectively. From there it would be easy to directly spawn the process by passing the ProcessStartInfo.