feat: Alpaca pydantic class for easy conversion, validation, and structured output generation

[CaelumF]

Description

Adds an alpaca pydantic class that allows for conversion to/from the Alpaca inference/training textual representation, and through pydantic, to/from json representations.

The expected way to generate this is however through structured output, though the other methods are useful for inference, training, and combining existing items from other sources.

Motivation and Context

Possibly closes #1184, though it doesn't provide a discoverable way to do this unless people are already familiar with Pydantic. Potentially a 1-liner function can be added to BaseMessage that shows how the message string can be easily converted, or maybe we do not need this to be particularly discoverable through the code APIs, if a larger theme of relying on pydantic classes for conversion is established as a recognisable pattern.

Types of changes

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds core functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (update in the documentation)
• Example (update in the folder of example)

Checklist

Go over all the following points, and put an x in all the boxes that apply.
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• I have read the CONTRIBUTION guide. (required)
• My change requires a change to the documentation.
• I have updated the tests accordingly. (required for a bug fix or a new feature)
• I have updated the documentation accordingly.

[Wendong-Fan]

Thanks @CaelumF , could you help resolve the conflict with master branch?