Vertex AI introduces Agents and Semantic Governance resources

This update adds over 500 new paths to the discovery document, primarily centered around two new resource types. The projects.locations.agents resource allows for the programmatic creation, retrieval, and deletion of AI agents. Additionally, the projects.locations.semanticGovernancePolicyEngine resource has been introduced as a singleton for managing semantic governance policies at the location level, supporting get and update (upsert) operations. Many of these new operations, such as agent creation and policy updates, are implemented as Long Running Operations (LROs).