feat: support single-agent Isaac Gym and multi-agent multi-goal

[Gaiejj]

This pull request would support single-agent Isaac Gym and multi-agent multi-goal environment interface.

[codecov]

Codecov Report

Merging #55 (8815909) into main (35c0804) will decrease coverage by 0.51%.
Report is 3 commits behind head on main.
The diff coverage is 55.55%.

❗ Current head 8815909 differs from pull request most recent head 2062e75. Consider uploading reports for the commit 2062e75 to get more accurate results

@@            Coverage Diff             @@
##             main      #55      +/-   ##
==========================================
- Coverage   91.62%   91.12%   -0.51%     
==========================================
  Files          26       27       +1     
  Lines        5267     5576     +309     
==========================================
+ Hits         4826     5081     +255     
- Misses        441      495      +54     

Files Changed	Coverage Δ
safepo/common/env.py	53.65% <20.00%> (-5.25%)	⬇️
safepo/common/model.py	97.39% <100.00%> (ø)

... and 16 files with indirect coverage changes

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