FdoSecrets: reject setting refs via the API

[Aetf]

Fixes #6802. Fixes #3992.

Check for references when setting fields calling from DBus. It still resolves references, though, so it's possible to manually create entries with references in KPXC UI.

Also added is a read-only field when reporting back attributes. This addresses the only actionable suggestion in #3992.

Testing strategy

Add unit test

Type of change

• ✅ Bug fix (non-breaking change that fixes an issue)
• ✅ New feature (change that adds functionality)

[codecov-commenter]

Codecov Report

Merging #7043 (05673da) into develop (b6716bd) will increase coverage by 0.01%.
The diff coverage is 87.80%.

@@             Coverage Diff             @@
##           develop    #7043      +/-   ##
===========================================
+ Coverage    63.61%   63.63%   +0.01%     
===========================================
  Files          330      330              
  Lines        41807    41822      +15     
===========================================
+ Hits         26595    26610      +15     
  Misses       15212    15212              

Impacted Files	Coverage Δ
src/fdosecrets/objects/Item.cpp	85.61% <87.80%> (+0.43%)	⬆️
src/fdosecrets/dbus/DBusObject.h	100.00% <0.00%> (+7.69%)	⬆️

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