VertexAiSearchTool User Credentials

**Is your feature request related to a problem? Please describe.**
Unable to pass user credentials to VertexAiSearchTool

**Describe the solution you'd like**
Developing an agent that uses the built-in VertexAiSearchTool to retrieve an answer from Vertex AI Search datastore. To ensure the user only sees information that they are entitled to see, I want to invoke an OAuth2 credential flow so the identity used to perform the search is that of the user.

**Describe alternatives you've considered**
Using a custom function tool