Mark Registration sub types as data objects

[twyatt]

Improves toString output, especially on JS, which currently translates to [object Object] (rather than Register or Deregister).

[codecov]

Codecov Report

Attention: 18 lines in your changes are missing coverage. Please review.

Comparison is base (56a59f1) 57.25% compared to head (28c92a8) 57.97%.
Report is 165 commits behind head on main.

Files	Patch %	Lines
koap/src/commonMain/kotlin/Message.kt	40.00%	12 Missing ⚠️
koap/src/commonMain/kotlin/Debug.kt	25.00%	3 Missing ⚠️
koap/src/commonMain/kotlin/Decoder.kt	66.66%	2 Missing ⚠️
koap/src/commonMain/kotlin/ByteArrayReader.kt	66.66%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #239      +/-   ##
============================================
+ Coverage     57.25%   57.97%   +0.71%     
  Complexity       11       11              
============================================
  Files             8        8              
  Lines           613      652      +39     
  Branches        145      144       -1     
============================================
+ Hits            351      378      +27     
- Misses          223      235      +12     
  Partials         39       39              

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