export method to build AvailableCommands packet for reuse

[HashimTheArab]

This PR moves the logic to build the AvailableCommands package out of session and into a public method in the cmd package.

This allows external callers to reuse this method instead of having to duplicate all of the code

[Sandertv]

Would it not make more sense to make this a public function in the session package? Since it's so directly tied to the protocol?
I would not be so ready to put this in the cmd package since it is quite susceptible to protocol changes.

[HashimTheArab]

Fair enough

[HashimTheArab]

Changes applied