fix: preserve tags and custom fields in metadata

[nogringo]

• Add tags field to Metadata entity
• Add rawContent field to preserve original JSON
• Update fromEvent() to store tags and rawContent
• Update toEvent() to merge with original JSON
• Add helper methods setCustomField() and getCustomField()
• Fix constructor to use mutable list for tags

Fixes #438

[codecov]

Codecov Report

❌ Patch coverage is 92.98246% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.41%. Comparing base (121325c) to head (06e1d6c).
⚠️ Report is 16 commits behind head on master.

Files with missing lines	Patch %	Lines
...ckages/ndk/lib/domain_layer/entities/metadata.dart	92.92%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #465      +/-   ##
==========================================
+ Coverage   76.01%   76.41%   +0.39%     
==========================================
  Files         152      152              
  Lines        6204     6291      +87     
==========================================
+ Hits         4716     4807      +91     
+ Misses       1488     1484       -4     

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[1-leo]

String? get name => content['name'] as String?;
getters are executed every time, because metadata is access frequently this could add up.
Can we cache them? Acessing custom properties should still be possible

[nogringo]

Should we update toJson and fromJson ? It will be a braking change.