Support AuthenticatedAgentTool

** Please make sure you read the contribution guide and file the issues in the right place. **
[Contribution guide.](https://google.github.io/adk-docs/contributing-guide/)

**Is your feature request related to a problem? Please describe.**
I am trying to build Agent with ReAct planner that uses Agents as Tools. There is no concept of authenticated AgentTool yet AgentTool is run as totally separate session meaning it does not generate events for authentication (or it genrates but they are not passed to main runner). Tools quite often are very technical or blend business and technical params that agent need to collect. Abstracting google toolset tools as agent which clarifies defaults and add more clarification on how to properly use distinct endpoints is more efficient way to have functional agent. Problem is that there is no way to authorize agent to act on user behalf on systems covered by that subagents used as tools.

**Describe the solution you'd like**
AuthorizedAgentTool with auth_config where we specify credentials_key where token is located or auth_credentials and auth_Schema wit hdetails how parent agent configured to use AuthenticatedAgentTool should collect.

**Describe alternatives you've considered**
NA

**Additional context**
Add any other context or screenshots about the feature request here.