chore: add cors endpoint filter

[JayDwee]

Summary

Checklist

• [ ] This PR fixes an issue.
• [ ] This PR adds something new (e.g. new method or parameters).
• [ ] This PR is a breaking change (e.g. methods or parameters removed/renamed)
• [ ] This PR is not a code change (e.g. documentation, README, ...)

• [ ] Have you tested the changes? (pytest & dpytest)
• [x] Have you followed PEP-8 for naming and styling?
• [x] Has your code been properly documented with RestructuredText docstrings?
• [ ] Have you added your changes to CHANGELOG.md under the [Unreleased] heading?
• [ ] If your code added new bot commands, have you updated documentation.json?

• [x] All of this code is your own, or follows the author repo's licence.

[codecov[bot]]

Codecov Report

Merging #410 (7491de5) into master (52f52c0) will decrease coverage by 0.01%. The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master     #410      +/-   ##
==========================================
- Coverage   87.35%   87.35%   -0.01%     
==========================================
  Files         123      123              
  Lines        8922     8926       +4     
==========================================
+ Hits         7794     7797       +3     
- Misses       1128     1129       +1     

Flag	Coverage Δ
unittests	87.35% <80.00%> (-0.01%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
koalabot.py	75.43% <66.66%> (-0.24%)	:arrow_down:
koala/env.py	82.35% <100.00%> (+1.10%)	:arrow_up: