.Net: Introduce Grouding with Google Search configuration

[scherervictor]

Motivation and Context

This PR is enables the configuration of Grouding with Google Search in the Gemini API.

Description

• Added grounding_config that receives google_search object
• Implemented a validation to add googleSearch under tools object if configured.
• Adding proper deserialization of possible values returning after using Grouding.
• Included unit tests to validate the functionality for GroudingConfig and GoogleSearch.

Contribution Checklist

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

Implements #11735

[scherervictor]

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