[Add] Added function calling support to GeminiClient

[arjun-g]

Why are these changes needed?

I have added function calling ability to GeminiClient so that we can use autogen tools with Gemini as well.

Related issue number

Completes "Function Calling" roadmap item in #2387

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[gitguardian]

⚠️ GitGuardian has uncovered 3 secrets following the scan of your pull request.

Please consider investigating the findings and remediating the incidents. Failure to do so may lead to compromising the associated services or software components.

🔎 Detected hardcoded secrets in your pull request

GitGuardian id	GitGuardian status	Secret	Commit	Filename
10493810	Triggered	Generic Password	7a9309e	notebook/agentchat_pgvector_RetrieveChat.ipynb	View secret
10493810	Triggered	Generic Password	7a9309e	notebook/agentchat_pgvector_RetrieveChat.ipynb	View secret
10493810	Triggered	Generic Password	7a9309e	notebook/agentchat_pgvector_RetrieveChat.ipynb	View secret

🛠 Guidelines to remediate hardcoded secrets

1. Understand the implications of revoking this secret by investigating where it is used in your code.
2. Replace and store your secrets safely. Learn here the best practices.
3. Revoke and rotate these secrets.
4. If possible, rewrite git history. Rewriting git history is not a trivial act. You might completely break other contributing developers' workflow and you risk accidentally deleting legitimate data.

To avoid such incidents in the future consider

• following these best practices for managing and storing secrets including API keys and other credentials
• install secret detection on pre-commit to catch secret before it leaves your machine and ease remediation.

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[arjun-g]

I am learning python just for the past few days. Kindly give me feedback on what I can improve in this.

[arjun-g]

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[arjun-g]

Closing due to GitGuardian error. Raised another PR #2793