[projmgr] Add RPC method DiscoverLayers (#1330)

[grasci-arm]

Relates to Open-CMSIS-Pack/cmsis-toolbox#419

Changes:

• Add RPC method DiscoverLayers similar to the command-line csolution list layers --update-idx.
• Move collection of possible layer variables configurations from emitter to worker for code re-use between command-line and rpc processing -> ProjMgrWorker::ElaborateVariablesConfigurations().
• Add test cases and update test utility functions.

Note: similar to the related command-line feature, solutions with undefined layer variables over multiple projects are currently not supported.

[github-actions]

Test Results

    3 files     21 suites   13m 32s ⏱️
  425 tests   425 ✅ 0 💤 0 ❌
1 275 runs  1 275 ✅ 0 💤 0 ❌

Results for commit f70c9fa.

[codecov]

Codecov Report

❌ Patch coverage is 85.78680% with 28 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.81%. Comparing base (4d190c6) to head (f70c9fa).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
tools/projmgr/src/ProjMgrRpcServer.cpp	75.86%	5 Missing and 9 partials ⚠️
tools/projmgr/src/ProjMgrWorker.cpp	89.55%	2 Missing and 5 partials ⚠️
tools/projmgr/test/src/ProjMgrRpcTests.cpp	79.16%	0 Missing and 5 partials ⚠️
tools/projmgr/src/ProjMgr.cpp	90.00%	0 Missing and 1 partial ⚠️
tools/projmgr/test/src/ProjMgrTestEnv.cpp	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2241      +/-   ##
==========================================
+ Coverage   66.76%   66.81%   +0.04%     
==========================================
  Files         181      181              
  Lines       38729    38820      +91     
  Branches    22729    22789      +60     
==========================================
+ Hits        25859    25937      +78     
- Misses       8079     8080       +1     
- Partials     4791     4803      +12     

Flag	Coverage Δ
projmgr-cov	80.94% <85.78%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
tools/projmgr/include/ProjMgr.h	100.00% <ø> (ø)
tools/projmgr/include/ProjMgrWorker.h	64.28% <ø> (ø)
tools/projmgr/src/ProjMgrCbuildIdx.cpp	95.85% <100.00%> (+2.19%)	⬆️
...ols/projmgr/test/src/ProjMgrGeneratorUnitTests.cpp	78.75% <100.00%> (-0.32%)	⬇️
tools/projmgr/test/src/ProjMgrUnitTests.cpp	80.23% <100.00%> (-0.01%)	⬇️
tools/projmgr/src/ProjMgr.cpp	80.44% <90.00%> (+0.19%)	⬆️
tools/projmgr/test/src/ProjMgrTestEnv.cpp	76.96% <83.33%> (+0.19%)	⬆️
tools/projmgr/test/src/ProjMgrRpcTests.cpp	57.69% <79.16%> (+0.86%)	⬆️
tools/projmgr/src/ProjMgrWorker.cpp	87.99% <89.55%> (+0.02%)	⬆️
tools/projmgr/src/ProjMgrRpcServer.cpp	70.72% <75.86%> (+0.90%)	⬆️

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