[OpAMP] Expose public CustomMessageMessage and CustomCapabilitiesMessage

[RassK]

Towards #3613

Changes

Exposes two related messages to public:

• CustomMessageMessage
• CustomCapabilitiesMessage

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.72%. Comparing base (b6723bb) to head (43417b0).
⚠️ Report is 5 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3681      +/-   ##
==========================================
+ Coverage   71.58%   71.72%   +0.14%     
==========================================
  Files         455      445      -10     
  Lines       17700    17653      -47     
==========================================
- Hits        12670    12662       -8     
+ Misses       5030     4991      -39     

Flag	Coverage Δ
unittests-Instrumentation.Cassandra	?
unittests-OpAmp.Client	80.84% <100.00%> (+0.10%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...lient/Messages/Custom/CustomCapabilitiesMessage.cs	100.00% <100.00%> (ø)
...Amp.Client/Messages/Custom/CustomMessageMessage.cs	100.00% <100.00%> (ø)

... and 11 files with indirect coverage changes

🚀 New features to boost your workflow:

• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.