[INFRA] Put SDSL into contrib

[eseiler]

Creates an amalgamated version of sdsl in contrib and removes dependency on the sdsl repo.

• sdsl-v3 is neither maintained nor widely packaged, the includes and namespaces clash with the more widely packages sdsl-v2
• Because of that, we vendor/ship the sdsl anyway when building packages
• After this change, we can switch to using find_package for cereal and have a somewhat mainstream CMake way of handling dependencies
• Even though the header is 1 MB big, shipping the sdsl include directory takes 2 MB. A checkout of the sdsl is 5 MB. seqan3 checkout grows from 18 to 19 MB, the include from 5 to 6 MB.

[seqan-actions]

Documentation preview available at https://docs.seqan.de/preview/seqan/seqan3/3334

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.11%. Comparing base (ee18011) to head (34bc3da).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3334   +/-   ##
=======================================
  Coverage   98.11%   98.11%           
=======================================
  Files         270      270           
  Lines       11913    11913           
  Branches      104      104           
=======================================
  Hits        11688    11688           
  Misses        225      225           

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