Rysweet 4702 add microsoftautogenagenthost package

[rysweet]

• remove sample Backend Package
• Move Host.cs and Aspire dependency out of Runtime.Grpc
• adds AgentHost package that is used for building host

Why are these changes needed?

reduces reps and moves backend from samples to src

Related issue number

closes #4702

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

Copilot reviewed 6 out of 14 changed files in this pull request and generated no comments.

Files not reviewed (8)

• dotnet/AutoGen.sln: Language not supported
• dotnet/samples/Hello/Backend/Backend.csproj: Language not supported
• dotnet/samples/Hello/Hello.AppHost/Hello.AppHost.csproj: Language not supported
• dotnet/src/Microsoft.AutoGen/AgentHost/Microsoft.Autogen.AgentHost.csproj: Language not supported
• dotnet/src/Microsoft.AutoGen/AgentHost/Properties/launchSettings.json: Language not supported
• dotnet/src/Microsoft.AutoGen/Runtime.Grpc/Microsoft.AutoGen.Runtime.Grpc.csproj: Language not supported
• dotnet/samples/Hello/Backend/README.md: Evaluated as low risk
• dotnet/src/Microsoft.AutoGen/Core/Agent.cs: Evaluated as low risk

Comments suppressed due to low confidence (1)

dotnet/src/Microsoft.AutoGen/Extensions/Aspire/AspireHostingExtensions.cs:20

• Changing the method signature from IHostApplicationBuilder to WebApplicationBuilder is a breaking change and should be reviewed carefully.

public static WebApplicationBuilder AddServiceDefaults(this WebApplicationBuilder builder)