Add support for connecting to kubernetes pods using exec

Hi there,

We are using guacamole for connecting to kubernetes pods from inside a
cluster.
Current implementation is attaching to a running application, if it is not a shell you are not able to see a command prompt.

When guacd initiate a session it send a request using "attach" call, like
"/api/v1/namespaces/test/pods/test-pod/attach?stdin=true&stdout=true&tty=true".

There is another way to initiate a session, using call "exec" with parameter
"command",  "/api/v1/namespaces/test/pods/test-pod/exec?command=/bin/sh&stdin=true&stdout=true&tty=true".

This is important for us and I think it could be a useful feature and would
be great to add this to a codebase.
Thanks.