fix: introduce deck content to dbless config converter

[czeslavo]

What this PR does / why we need it:

Introduces ContentToDBLessConfigConverter used by UpdateStrategyInMemory to adapt decK's file.Content to match DBLess schema constraints.

It extracts the already existing conversions from UpdateStrategyInMemory (dropping Info field, cleaning up nulls in plugins' configs), and also adds cleanups for ConsumerGroups related fields (dropping Plugins[i].ConsumerGroup, Consumers[i].Group, ConsumerGroups[i].Plugins, ConsumerGroups[i].Consumers, filling relationships).

Also adds an example config for ConsumerGroup.

Which issue this PR fixes:

Fixes #4448.

[codecov]

Codecov Report

Patch coverage: 100.0% and project coverage change: -1.5% ⚠️

Comparison is base (7e567cc) 67.5% compared to head (25609f7) 66.0%.
Report is 1 commits behind head on main.

❗ Current head 25609f7 differs from pull request most recent head 4ad45a4. Consider uploading reports for the commit 4ad45a4 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #4453     +/-   ##
=======================================
- Coverage   67.5%   66.0%   -1.5%     
=======================================
  Files        160     156      -4     
  Lines      18804   18722     -82     
=======================================
- Hits       12699   12372    -327     
- Misses      5343    5631    +288     
+ Partials     762     719     -43     

Files Changed	Coverage Δ
internal/dataplane/deckgen/deckgen.go	64.0% <ø> (+8.5%)	⬆️
internal/dataplane/sendconfig/inmemory.go	72.0% <100.0%> (-17.7%)	⬇️
internal/dataplane/sendconfig/inmemory_schema.go	100.0% <100.0%> (ø)
internal/dataplane/sendconfig/strategy.go	100.0% <100.0%> (ø)

... and 35 files with indirect coverage changes

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