feat: migrate to Kiota client and self-made openapi spec

[Agash]

• hand-written openapi spec in openapi/goaffpro-canonical.yaml since the openapi definitions on the official page show significatn spec drift and are thus unreliable.
• replaced generator with Kiota-based client generation from openapi/goaffpro-canonical.yaml now that it's a well defined spec.
• refactored wrapper/event surface to emit generated feed models directly
• added observer support for more feeds (orders, traffic, payouts, products, transactions; rewards remains obsolete/unstable)
• retargeted projects to net9.0 and net10.0 (removed .NET8, will remove .NET9 as support ends this year).
• upgraded example app into an interactive playground
• annotated known unstable/parity endpoints directly in yaml spec

checks

• dotnet build GoAffPro.Client.slnx --configuration Release --no-restore
• dotnet test GoAffPro.Client.slnx --configuration Release --no-build --filter "Category!=Integration"
• dotnet test tests/GoAffPro.Client.IntegrationTests/GoAffPro.Client.IntegrationTests.csproj --filter "Category=Integration"
• dotnet build src/GoAffPro.Client.Generated/GoAffPro.Client.Generated.csproj -v minimal (Kiota regeneration check)

Notes

• known api issues remain (documented in spec comments):
  • /user/feed/products (timeouts, tried 30s and 90s multiple times, might be related to my empty account)
  • /user/feed/rewards (404/non-JSON)
  • /user/feed/transactions (500/non-JSON )
  • /user/payouts/pending tracked as parity endpoint also seen under /sdk/user/* not in official api spec visible