feat: add timestamp util

[shoothzj]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 18.18182% with 9 lines in your changes missing coverage. Please review.

Project coverage is 29.74%. Comparing base (fba8bf3) to head (0e0c0a6).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
opcua/util/timestamp.go	0.00%	5 Missing ⚠️
opcua/enc/decoder.go	0.00%	2 Missing ⚠️
opcua/enc/special_encoder.go	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #30      +/-   ##
==========================================
- Coverage   29.84%   29.74%   -0.10%     
==========================================
  Files          25       26       +1     
  Lines        1565     1570       +5     
==========================================
  Hits          467      467              
- Misses        957      962       +5     
  Partials      141      141              

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