feat: Implement automatic tool discovery for config-based agents

[copybara-service]

feat: Implement automatic tool discovery for config-based agents

This change allows ConfigAgentLoader to scan agent YAML files for tool names that are fully qualified Java class or method/field references. It automatically registers these tools in the ComponentRegistry.

The following patterns for tool names in the YAML are supported:

1. Class Name: e.g., com.example.MyTool

   • LlmAgent will instantiate this class.
   • If args are provided in the YAML, a static fromConfig(ToolArgsConfig) method must exist.
   • Otherwise, the class must have a default constructor.
   • This is typically used for tools extending BaseTool.

2. Fully Qualified Static Field: e.g., com.example.MyTool.MY_TOOL_INSTANCE

   • LlmAgent will resolve the static field MY_TOOL_INSTANCE from the class com.example.MyTool.
   • The field must be of a type assignable to BaseTool.

3. Registry Name: e.g., my_custom_tool

   • The tool must be pre-registered in the ComponentRegistry.
   • A custom ComponentRegistry can be specified via the system property -Dregistry=com.example.CustomRegistry.

Usage Example:

# Navigate to the example directory
cd maven_plugin/examples/custom_tools

# Clean, compile, and start the server
mvn clean compile google-adk:web \
  -Dagents=config_agents \
  -Dregistry=com.example.CustomDieRegistry