Add mac support for Mujoco inference

[danielwright-lgtm]

Currently the unitree_sdk2 package is not supported for Mac. This PR instead installs the unitree_sdk2_python package so that inference with Mujoco can work on a Mac.

Description of changes:

1. Remove the install of unitree_sdk2 and replace it with unitree_sdk2_python for Macbooks.
2. Add a mac specific bridge file which receives commands using unitree_sdk2_python.
3. Add logic to choose whether to use the original bridge file or the mac specific file.
4. Add an interface file which sends commands from the policy using unitree_sdk2_python.

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