.NET: Background responses sample (#1831)
* add samples that demonstrate background responses with function calling and agent state persistence
* update readme
* update sample description
* rename sample project
* remove unnecessary namespace and ysed task delay instead thread sleep generated by ai
* change names
* Update dotnet/samples/GettingStarted/Agents/Agent_Step20_BackgroundResponsesWithToolsAndPersistence/AgentStateStore.cs
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
* simplify sample and address pr review comments
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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
SergeyMenshykh