[OpAMP.Client] Add initial public client

[RassK]

Changes

Adds initial public client with features:

• Create the client
• Modify client's options
• Start the client and services

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

❌ Patch coverage is 23.07692% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.39%. Comparing base (a4a0182) to head (e2178e2).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/OpenTelemetry.OpAmp.Client/OpAmpClient.cs	0.00%	12 Missing ⚠️
...metry.OpAmp.Client/Settings/OpAmpClientSettings.cs	33.33%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3103      +/-   ##
==========================================
- Coverage   69.91%   65.39%   -4.53%     
==========================================
  Files         421      409      -12     
  Lines       16602    15574    -1028     
==========================================
- Hits        11607    10184    -1423     
- Misses       4995     5390     +395     

Flag	Coverage Δ
unittests-OpAmp.Client	67.16% <23.07%> (-2.35%)	⬇️

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

Files with missing lines	Coverage Δ
...penTelemetry.OpAmp.Client/Internal/FrameBuilder.cs	64.44% <ø> (ø)
...Telemetry.OpAmp.Client/Internal/FrameDispatcher.cs	80.00% <ø> (ø)
...nt/Internal/Services/Heartbeat/HeartbeatService.cs	74.07% <ø> (ø)
...lemetry.OpAmp.Client/Settings/HeartbeatSettings.cs	100.00% <100.00%> (ø)
...metry.OpAmp.Client/Settings/OpAmpClientSettings.cs	33.33% <33.33%> (ø)
src/OpenTelemetry.OpAmp.Client/OpAmpClient.cs	0.00% <0.00%> (ø)

... and 89 files with indirect coverage changes

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[Kielek]

LGTM