feat: Create GEPARootAgentOptimizer

This is a generalization of the GepaRootAgentPromptOptimizer which adds support for optimizing skill instructions alongside the agent's core instructions. In the future this optimizer can be extended to add support for optimizing additional aspects of the ADK agent such as tool specifications.

Co-authored-by: Keyur Joshi <keyurj@google.com>
PiperOrigin-RevId: 929431849

Author: KPJoshi