Add autokey cipher and tests

[bennybebo]

• [x] I have read CONTRIBUTING.md.
• [x] This pull request is all my own work -- I have not plagiarized it.
• [x] All filenames are in PascalCase.
• [x] All functions and variable names follow Java naming conventions.
• [x] All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• [x] All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 54.61%. Comparing base (f34fe4d) to head (bd39ead).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5569      +/-   ##
============================================
+ Coverage     54.56%   54.61%   +0.05%     
- Complexity     3413     3418       +5     
============================================
  Files           535      536       +1     
  Lines         15383    15406      +23     
  Branches       2935     2937       +2     
============================================
+ Hits           8393     8414      +21     
- Misses         6637     6638       +1     
- Partials        353      354       +1     

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