prompty: expose the 'model' property for use in flex flows

[ianchi]

Description

Useful in flex flows for instance to adapt the external logic to some conditions (.i.e if the api is chat or completion, if it is streaming or not, etc).

Solves #3813

All Promptflow Contribution checklist:

• The pull request does not introduce [breaking changes].
• CHANGELOG is updated for new features, bug fixes or other significant changes.
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Testing Guidelines

• Pull request includes test coverage for the included changes.

[github-actions]

Hi, thank you for your interest in helping to improve the prompt flow experience and for your contribution. We've noticed that there hasn't been recent engagement on this pull request. If this is still an active work stream, please let us know by pushing some changes or leaving a comment.

[ianchi]

Hi, any chance to advance with this?

[ianchi]

Hi, any chance to advance with this?

[github-actions]

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