chore: Add support for optional parameters

[dishaprakash]

This PR introduces full support for optional parameters, corresponding to the feature added to the Toolbox server (googleapis/genai-toolbox#617).

This change is similar to the support of optional parameters in the python SDK (googleapis/mcp-toolbox-sdk-python#290)

Overview of changes

1. ParameterSchema : The struct now includes a field called required, allowing the tools to have optional parameters
2. Parameter validation : The validation check now will not throw an error if the value is nil, given that the parameter is optional.
3. Cleaner payload: Any parameter with a null value will be omitted from the payload