feat(elb): add new check elb_desync_mitigation_mode

[MarioRgzLpz]

Context

Add a new check to ensure Classic Load Balancers are configured with either the defensive or strictest desync mitigation mode to prevent HTTP desynchronization issues. This control fails if the desync mitigation mode does not use one of these modes.

HTTP desync vulnerabilities can lead to serious security risks like request smuggling, credential hijacking, and unauthorized command execution. Enabling defensive or strictest desync mitigation modes helps protect applications from these risks by safeguarding against potential desynchronization attacks.

Description

Changed elb service to add a new attribute desync_mitigation_mode to LoadBalancer model. Added a new check elb_desync_mitigation_mode with respective unit tests.

Checklist

• Are there new checks included in this PR? Yes
  • If so, do we need to update permissions for the provider? No.
• Review if the code is being covered by tests.
• Review if code is being documented following this specification https://github.com/google/styleguide/blob/gh-pages/pyguide.md#38-comments-and-docstrings
• Review if backport is needed.

License

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.62%. Comparing base (ffa29f2) to head (43d454c).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5500      +/-   ##
==========================================
+ Coverage   89.58%   89.62%   +0.03%     
==========================================
  Files        1073     1075       +2     
  Lines       33237    33287      +50     
==========================================
+ Hits        29777    29832      +55     
+ Misses       3460     3455       -5     

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