.Net: Feature declarative agents (#11347)

### Motivation and Context

This PR introduces declarative agent support by adding new abstractions
and definitions for configuring agents via YAML and by updating existing
agent constructs.
- New model, connection, tool, metadata, input, factory, definition, and
creation option classes have been added.
- The access modifier for the Template property in Agent has been
updated to public.
- Multiple samples for ChatCompletion, OpenAIAssistant, and Bedrock
agents demonstrate declarative agent creation via YAML.

### Description

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These notes will help understanding how your code works. Thanks! -->

### Contribution Checklist

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- [ ] The code builds clean without any errors or warnings
- [ ] 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
- [ ] All unit tests pass, and I have added new tests where possible
- [ ] I didn't break anyone 😄

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Co-authored-by: Chris <[email protected]>
Co-authored-by: Dmytro Struk <[email protected]>
Co-authored-by: westey <[email protected]>
Co-authored-by: Copilot <[email protected]>

Author: 5 people