fix(config): Move ObjectMapper to internal YamlObjectMapper class

[MikeGoldsmith]

Summary

Moves ObjectMapper configuration from package-private field in DeclarativeConfiguration to new public internal class YamlObjectMapper. Spring starter and other external code can now access the configured ObjectMapper without reflection.

Fixes #7843

Changes

• New class: YamlObjectMapper in internal package with singleton getInstance() method
• Refactored: DeclarativeConfiguration to use YamlObjectMapper.getInstance() instead of static MAPPER field
• Removed now-unused Jackson imports from DeclarativeConfiguration

Testing

• All existing tests pass

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.20%. Comparing base (f42ac7d) to head (258835a).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8075   +/-   ##
=========================================
  Coverage     90.20%   90.20%           
- Complexity     7593     7595    +2     
=========================================
  Files           841      842    +1     
  Lines         22913    22915    +2     
  Branches       2289     2289           
=========================================
+ Hits          20668    20670    +2     
  Misses         1529     1529           
  Partials        716      716           

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[jack-berg]

@zeitlinger / @trask is this still needed from the instrumentation side? The last comment in the issue indicates a preference for copy / paste rather than letting consumers leverage the same object mapper:

I personally prefer a bit of copy-paste over relying on internal APIs

My perspective: in addition to not relying on internal APIs, I don't like being in the business of having an API (public or internal) to access a jackson ObjectMapper. That's not our business.