Add initial ASF Celix security model

[pnoltes]

This PR adds the initial Apache Celix Security Model (SECURITY.md) following ASF security guidelines (https://www.apache.org/security/).

This document sets up the project's security rules and explains how to report security issues, following Apache Software Foundation requirements.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.58%. Comparing base (270c784) to head (2bd1fcd).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #849      +/-   ##
==========================================
+ Coverage   91.54%   91.58%   +0.04%     
==========================================
  Files         235      235              
  Lines       28787    28787              
==========================================
+ Hits        26353    26366      +13     
+ Misses       2434     2421      -13     

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[PengZheng]

LGTM