feat: add on_model_request_callback to plugins

[gmrnlg1971]

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[rohityan]

Hi @gmrnlg1971 , Thank you for your contribution! It appears you haven't yet signed the Contributor License Agreement (CLA). Please visit https://cla.developers.google.com/ to complete the signing process.
Could you also add a description and once the CLA is signed and description added, we'll be able to proceed with the review of your PR. Thank you!