Add new *.impl.* package convention, internal code with japicmp compatibility

[jack-berg]

Related to #6970.

We need a new stability tier for code that is public (for cross-module access within the OpenTelemetry project) but not intended for application developers — with full backwards-compatibility guarantees, unlike .internal..

As discussed in 4/23 java SIG, ApiUsageLogger from #8318 is an early candidate for this.

Others potential candidates:

• CompoenntId and other internal instrumentation utils
• ComponentRegistry and other internal SDK implementation utils (ThrottlingLogger, DaemonThreadfactory, GlobUtil, etc)
• GrpcExporter, HttpExporter, and other exporter impelemtnation utils
• And more

Options considered:

• .util.: rejected because Java developers associate "util" with user-facing packages (java.util.concurrent, etc.), which may mislead application developers into thinking the code is for them.
• .internal.shared. / .internal.stable.: rejected because these live under .internal., which the project has established as meaning unstable. Carving out a stable sub-package would contradict that convention and require fiddly japicmp exclusion changes.
• .moduleinternal. / .componentapi.: considered but rejected as too verbose or without prior art in the Java ecosystem.
• .impl. — chosen because it is a reasonably well-understood Java idiom signaling "implementation detail, not user-facing API", it does not conflict with any existing OTel package conventions, and it falls outside the current japicmp exclusion patterns (*.internal / .internal.) so compatibility checking applies automatically with no build config changes.

Classes in .impl. packages must include standard javadoc boilerplate:

This class is not intended for use by application developers. Its API is stable and will not be changed or removed in a backwards-incompatible manner.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.27%. Comparing base (fdb8194) to head (dec019e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8325   +/-   ##
=========================================
  Coverage     90.27%   90.27%           
  Complexity     7693     7693           
=========================================
  Files           850      850           
  Lines         23207    23207           
  Branches       2356     2356           
=========================================
  Hits          20951    20951           
  Misses         1530     1530           
  Partials        726      726           

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[trask]

I like .impl

I assume there's nothing worth adding to VERSIONING.md since it's just a normal stable class from that perspective?