Move vector APIs to LiteDB.Vector extension surface

[JKamsker]

Summary

• remove vector query/index overloads from the core interfaces and keep the concrete types delegating through obsolete helpers
• add the LiteDB.Vector namespace with extension methods plus relocated VectorIndexOptions/VectorDistanceMetric
• update engine/client callers for the new namespace and add tests that exercise the extension surface

Testing

• dotnet test LiteDB.Tests/LiteDB.Tests.csproj

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https://chatgpt.com/codex/tasks/task_e_68d7ef1f9544832ab0d52012492d8635