Python: Introduce the single, non-group chat completion agent

[moonbox3]

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.

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 #7079

Contribution Checklist

• [X] The code builds clean without any errors or warnings
• [X] The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• [X] All unit tests pass, and I have added new tests where possible
• [X] I didn't break anyone :smile:

[markwallace-microsoft]

Python 3.10 Test Coverage Report •

File	Stmts	Miss	Cover	Missing
TOTAL	6986	380	95%

report-only-changed-files is enabled. No files were changed during this commit :)

Python 3.10 Unit Test Overview

Tests	Skipped	Failures	Errors	Time
1782	1 :zzz:	0 :x:	0 :fire:	25.108s :stopwatch: