Drop dependency on EF Core internals

[bkoelman]

This fixes a warning in one of the samples when using EF Core 9 (preview 1), to not depend on EF Core internals. The fix also avoids the need for dealing with a breaking change in EF Core 9, and it optimizes to build the model only once at startup.

Furthermore, this PR reduces cohesion by internally depending on IReadOnlyModel instead of IModel.

QUALITY CHECKLIST

• Changes implemented in code
• Complies with our contributing guidelines
• N/A: Adapted tests
• N/A: Documentation updated

[codecov]

Codecov Report

Attention: Patch coverage is 98.03922% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 90.75%. Comparing base (06d6b18) to head (3bf9157).

Files	Patch %	Lines
...ionToSql/DataModel/FromEntitiesDataModelService.cs	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1492   +/-   ##
=======================================
  Coverage   90.74%   90.75%           
=======================================
  Files         344      344           
  Lines       11003    11006    +3     
  Branches     1805     1806    +1     
=======================================
+ Hits         9985     9988    +3     
  Misses        669      669           
  Partials      349      349           

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