enhance(arbiter-engine): remove <E, A> generics from World

[Autoparallel]

Give an overview of the tasks completed
We wanted to not have the World struct have generics that come from the Engine<E,A> inside of the Agent struct. This PR fixes this by introducing a object-safe trait Entity that solely provides the interface to do Agent::run(). Minimal change with no breaks to tests. Changed the agent collection in World from Vec<Agent> to HashMap<String, Agent> while I was at it.

Link to issue(s) that this PR closes
Closes #747

[codecov]

Codecov Report

Attention: 15 lines in your changes are missing coverage. Please review.

Comparison is base (c851c5e) 52.51% compared to head (c3a89c8) 52.45%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
arbiter-engine/src/world.rs	0.00%	11 Missing ⚠️
arbiter-engine/src/agent.rs	0.00%	3 Missing ⚠️
arbiter-engine/src/examples.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #748      +/-   ##
==========================================
- Coverage   52.51%   52.45%   -0.06%     
==========================================
  Files          36       36              
  Lines        7293     7301       +8     
==========================================
  Hits         3830     3830              
- Misses       3463     3471       +8     

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[0xJepsen]

lgtm