Fix: google grounding is not working

[maximetassy]

Google grounding is not working

Problem

Google Grounding tool does not work with model >= 2.0.
The implementation use googleSearch tool which does not seem to be the way google wants to go with this tool.
As stated here, we should use google_search with an empty object.

Also, some attributes of GroundingMetadata are nullable in the documentation and the app crash if they are not defined.

Solution

• Update GoogleGrounding class to use the new way of GoogleGrounding tool declaration.
• Make groundingChunks, groundingSupports, webSearchQueries and retrievalMetadata nullable

[Plytas]

Thanks for this @maximetassy!