Use customtool's get_tool_definition to remove duplication

[jeffxtang]

What does this PR do?

Current examples would cause a lot of unnecessary painful duplication when a bunch of custom tools are expected while dealing with a real use case.

Also added pip install -U httpx==0.27.2 to avoid a httpx proxies error when running in an env with 0.28 or higher of httpx installed by default.

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• Addresses issue (#issue)

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• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Updated relevant documentation.
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[ashwinb]

thanks!