Dotnet: Add modelServiceId support to SemanticKernelAgent

[cedricmendelin]

The SemanticKernelAgent class has been updated to include an optional modelServiceId parameter, allowing the specification of a service ID for the model.

Why are these changes needed?

Currently, SemanticKernelAgent uses the parameterless method for resolving IChatCompletionSerivce. This will fail, when multiple models are registered in the Kernel.

To support different models registered in the Kernel, I adopted the resolving of the IChatCompletionSerivce within the SemanticKernelAgent with an optional parameter. When it is not set, I resolve the default instance, otherwise, I use the optional parameter as a servide id for resolving the IChatCompletionSerivce service.

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