updated: added multi-robot scalability with dynamic namespaces.

[ThatGuySoham]

Hey BCR team, I was working on my own project to dive deeper into the world of robotics software and I stumbled across bcr_bot. Noticing how similar the works were, I couldnt keep myself from trying to collab the codebase.

Here are the 2 architecture changes I am proposing:

1. gz.launch.py - Added namespace arguments as suggested in the issue.
2. bcr_bot_gz_spawn.launch.py - dynamic remapping for all nodes, gazebo models, ros bridges

And heres a brief of the testing:

1. Built locally and spawned using "namespace:=alpha_bot"
2. Verified clean topic isolation ("/alpha_bot/cmd_vel, /alpha_bot/joint_states", etc) and a collision-free TF tree.