Add new Multi-Agent environments

[Hunter-Unity]

Proposed change(s)

This adds 2 new envs that demonstrate multi-agent behaviors. The included envs are:

1. PushBlockCollab: Agents must work together in order to push heavy blocks to the goal.
2. ZombiePushBlock: An agent must learn to sacrifice itself for the good of the group. Get the key from the zombie to unlock the block. Push the unlocked block to the goal.

Types of change(s)

• Bug fix
• New feature
• Code refactor
• Breaking change
• Documentation update
• Other (please describe)

Checklist

• Added tests that prove my fix is effective or that my feature works
• Updated the changelog (if applicable)
• Updated the documentation (if applicable)
• Updated the migration guide (if applicable)

Other comments

[GoingMyWay]

The new environments look good, will you merge them?