[rfc][confmap]: add two new approaches for merging lists across configurations

[VihasMakwana]

Following up on my meeting with @mx-psi, @dmitryax and @evan-bradley, I'm adding two more approaches we discussed during the meet for docs/rfcs/configuration-merging-strategy.md.

1. Specifying the lists to merge via command line flags:
   • This approach adds separate top-level CLI flags to control the merging behaviour.
2. Specifying the lists to merge in the configuration itself:
   • This approac uses a dedicated top-level merge: section that declares which paths in that file should be merged into the running configuration

[codspeed-hq]

Merging this PR will not alter performance

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

✅ 7 untouched benchmarks
⏩ 76 skipped benchmarks¹

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_{Comparing VihasMakwana:configmap-rfc-update (d8b8738) with main (28265fc)}

Footnotes

1. 76 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.27%. Comparing base (28265fc) to head (0407951).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #15107   +/-   ##
=======================================
  Coverage   91.27%   91.27%           
=======================================
  Files         699      699           
  Lines       44756    44756           
=======================================
+ Hits        40850    40853    +3     
+ Misses       2766     2764    -2     
+ Partials     1140     1139    -1     

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