…oft#7116)

### Motivation and Context

Following in the steps of dotnet, we're beginning to add agent support
to Python. For an initial implementation the focus is on non-grouped,
chat completion agents. The first pass is covering a ChatCompletionAgent
that allows the user to define a ChatCompletion Service as part of the
SK Kernel, and then reference that service in an agent implementation.

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### Description

The PR:
- Adds support for a standard chat completion agent
- Adds the required abstractions to allow the single chat completion
agent to work
- Introduces a new agent concept sample showing how to invoke the agent
for a streaming and non-streaming context.
- Adds unit tests to cover the new code
- Closes microsoft#7079 


![image](https://github.com/microsoft/semantic-kernel/assets/35585003/8fa1f248-52b2-41b9-85ac-38ac80a05a68)

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### Contribution Checklist

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- [X] The code builds clean without any errors or warnings
- [X] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [X] All unit tests pass, and I have added new tests where possible
- [X] I didn't break anyone 😄