chore: add decimicrosecond datetime

[noaccOS]

import decimicrosecond datetime from astarte-platform/astarte#1142

this way it can be used in data_access's datetime for cast and split_submillis

[codecov]

Codecov Report

❌ Patch coverage is 0% with 26 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.29%. Comparing base (7ba3d8f) to head (ca2c716).
⚠️ Report is 9 commits behind head on release-1.2.

Files with missing lines	Patch %	Lines
lib/astarte_core/decimicrosecond_datetime.ex	0.00%	26 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           release-1.2     #119      +/-   ##
===============================================
- Coverage        76.53%   74.29%   -2.25%     
===============================================
  Files               50       51       +1     
  Lines              861      887      +26     
===============================================
  Hits               659      659              
- Misses             202      228      +26     

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