2578 stub agent plugins endpoint

[cakekoa]

What does this PR do?

Fixes #2578.

Adds the /api/agent-plugins resource.

PR Checklist

• Have you added an explanation of what your changes do and why you'd like to include them?
• Is the TravisCI build passing?
• Was the CHANGELOG.md updated to reflect the changes?
• Was the documentation framework updated to reflect the changes?
• Have you checked that you haven't introduced any duplicate code?

Testing Checklist

• Added relevant unit tests?
• Have you successfully tested your changes locally? Elaborate:

  Tested by {Running the Monkey locally with relevant config/running Island/...}

• If applicable, add screenshots or log transcripts of the feature working

[codecov]

Codecov Report

Base: 62.68% // Head: 62.73% // Increases project coverage by +0.04% 🎉

Coverage data is based on head (d2ea9d3) compared to base (a2a588a).
Patch has no changes to coverable lines.

❗ Current head d2ea9d3 differs from pull request most recent head c1b9600. Consider uploading reports for the commit c1b9600 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2611      +/-   ##
===========================================
+ Coverage    62.68%   62.73%   +0.04%     
===========================================
  Files          353      354       +1     
  Lines        10792    10800       +8     
===========================================
+ Hits          6765     6775      +10     
+ Misses        4027     4025       -2     

Impacted Files	Coverage Δ
...monkey/monkey_island/cc/resources/agent_plugins.py	100.00% <0.00%> (ø)
monkey/monkey/monkey_island/cc/app.py	75.86% <0.00%> (+0.20%)	⬆️
.../monkey/tests/unit_tests/monkey_island/conftest.py	95.65% <0.00%> (+4.34%)	⬆️

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