[systemd] add systemd receiver

[atoulme]

Description:
Add the systemd receiver to the Splunk distribution.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 33.25%. Comparing base (7f27a56) to head (771a41c).
⚠️ Report is 2 commits behind head on main.

❌ Your project check has failed because the head coverage (33.25%) is below the target coverage (35.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7143      +/-   ##
==========================================
+ Coverage   33.20%   33.25%   +0.04%     
==========================================
  Files         330      330              
  Lines       23325    23330       +5     
==========================================
+ Hits         7746     7759      +13     
+ Misses      14683    14676       -7     
+ Partials      896      895       -1     

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