Move admin space under user-configured namespace

[DenisBiryukov91]

Optional proposal to move admin space inside of configured namespace.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 71.19%. Comparing base (655c8d0) to head (ec7de60).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
zenoh/src/net/runtime/adminspace.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2197      +/-   ##
==========================================
- Coverage   71.28%   71.19%   -0.10%     
==========================================
  Files         375      375              
  Lines       60949    60949              
==========================================
- Hits        43450    43390      -60     
- Misses      17499    17559      +60     

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