increase allowed max_return_results (currently limited to 10)

**Is your feature request related to a problem? Please describe.**
I'm limited by VertexAISearch max_results being set to 10. I have a structured datastore, and would like the tool to be able to fetch more than 10 results, ex. 25-50

**Describe the solution you'd like**
Increase the max_results type spec, or allow additional parameters like offset

**Describe alternatives you've considered**
I've considered using filters in my implementation, but it's not as easy to avoid edge cases when setting filters vs. letting the retrieval layer cast a wider net.

I've considered using offset, but it is not available, and I've considered setting the relevance threshold, but it is also not available.

**Additional context**
I understand there is an issue with overloading the context window when fetching unstructured document snippets, so maybe a higher limit should be allowed for structured datastores.