.Net Agents - ChatCompletionAgent Pre-Graduation - #7900

### Motivation and Context
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Update public signatures / interface of `ChatCompletionAgent` in
preparation for removing experimental tag.

Fixes: microsoft#6037 
Fixes: microsoft#6523


### Description
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- Include `Kernel` and `KernelArguments` in agent invocation signatures
for `ChatCompletionAgent`
- Change `ChatCompletionAgent.ExeuctionSettings` to
`ChatCompletionAgent.KernelArguments`
- Mirror pattern for `OpenAIAssistantAgent`
- Integrate `Kernel` and `KernelArguments.ExecutionSettings` as
overrides for agent invocation
- Added new sample that explores the entire selection matrix:
`ChatCompletion_ServiceSelection`

### Contribution Checklist
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- [X] The code builds clean without any errors or warnings
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raises no violations
- [X] All unit tests pass, and I have added new tests where possible
- [X] I didn't break anyone 😄