feat: add multi agent environment

[Kakadus]

This enhances ns3-ai by adding a multi-agent environment. Its api is inspired by its single agent counterpart but has the advantage of a single function NotifyCurrentState. For extensibility, a delay can be configured for the result of the python agent.

We developed it to showcast several use-cases in context of LTE reinforcement learning. We will soon showcast our work at https://github.com/DEFIANCE-project

[OliverZim]

I am currently working on adding documentation and an example that utilizes the Ns3MultiAgentEnv.

[Asterohobit]

This looks promising. Will use this in the future.

[OliverZim]

I added documentation and examples for the multi-agent interface #dc35b6a. @Asterohobit How was your experience with the new feature so far?

[Asterohobit]

@Asterohobit How was your experience with the new feature so far?

Currently, I train single agents. I still use the multi-agent environment because of the fixes.