Replace manual concatenation with actual ASN.1 encoder

[Firehed]

While on its own the current implementation is perfectly stable due to fixed lengths, adding support for other formats will become highly problematic. Instead, replace the ASN.1 handling with a suitable library.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.51%. Comparing base (ef12eb8) to head (49006e6).

Additional details and impacted files

@@             Coverage Diff              @@
##               main      #78      +/-   ##
============================================
+ Coverage     78.01%   78.51%   +0.50%     
  Complexity      304      304              
============================================
  Files            32       32              
  Lines           955      954       -1     
============================================
+ Hits            745      749       +4     
+ Misses          210      205       -5     

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