[LibBlosc] Add a compcode keyword argument for BloscEncodeOptions

[nhz2]

I want to move away from using String for the 1.0 API.

https://github.com/JuliaAstro/ASDF.jl/blob/c18e875376358479c7380539f62f5ef298e957c6/src/ASDF.jl#L1848 can be updated to use compcode once this is released.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.53%. Comparing base (881e9dc) to head (0710602).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #94      +/-   ##
==========================================
+ Coverage   88.30%   97.53%   +9.22%     
==========================================
  Files          42        4      -38     
  Lines        2120       81    -2039     
==========================================
- Hits         1872       79    -1793     
+ Misses        248        2     -246     

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