Add ChatMessageExtensions with UsePromptCaching

[gcarney-ai]

Description

I have added a new file named ChatMessageExtensions with a single extension: UsePromptCaching. This change creates AdditionalProperties on the message if it is not already populated, and adds CachePoint ContentBlock. I used the Prompt caching for faster model inference for reference. The UsePrompCaching method takes an optional parameter CacheTTL which allows users to pass in either 5Min or 1Hr...defaulting to 5 min.

Motivation and Context

Prompt caching is a crucial piece for both latency and cost optimization, by placing it in a public extension, it enables discovery and reduces duplicate code written by consumers.

Testing

No existing test fixtures for this project. But I manually tested each case.

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have read the README document
• I have added tests to cover my changes
• All new and existing tests passed

License

• I confirm that this pull request can be released under the Apache 2 license