.Net Agents - Support Prompt Template - #8631

Motivation and Context

Support ability to paramaterize agent instructions (à la IKernelTemplate).

This also includes the ability to initialize an agent using a yaml based prompt-template.

Description

Core:

• Update ChatCompletionAgent and OpenAIAssistantAgent to support initialization via PromptTemplateConfig.
• Also allow both agent types to support semantic-kernel template format for string based Instructions
• Formalize templating contracts on KernelAgent (base class)
• Added GettingStartedWithAgents samples using yaml template: Step01 & Step08
• Added templating samples under Concepts/Agents to explore full range of templating patterns

Structural:

• Split off OpenAIAssistantCapabilities from OpenAIAssistantDefinition to clarify creation via PromptTemplateConfig
• Re-ordered method parameters for OpenAIAssistant.CreateAsync and RetrieveAsync to rationalize functional grouping and optionality.
• Externalized internal AssistantCreationOptionsFactory (from private OpenAIAssistant method) for clarity and ease of testing
• Persisting template-format as part of assistant metadata for retrieval case (in the event of any ambiguity)

Additionally:

• Updated/added comments where appropriate
• Updated sample conventions (argument labels, explicit types)
• Updated all call sites for creating OpenAIAssistantAgent (due to parameter ordering)
• Added test coverage where able

Contribution Checklist

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