[Feat]: Add tags field to AgentCard for agent categorization

### Is your feature request related to a problem? Please describe.

_No response_

### Describe the solution you'd like

Add an optional `agent_type` field (string) to the `AgentCard` message that allows agents to declare their type/category.

Example values: "llm", "workflow", "assistant", "retrieval", "code-executor", or any custom value.

Benefits:
- Helps consumers quickly identify the kind of agent they are interacting with
- Enables filtering and sorting in agent registries and catalogs
- Flexible string type (not enum) allows any custom categorization without protocol changes
- Optional field ensures backward compatibility

Changes required:
- Add `optional string agent_type = 20;` to AgentCard in `a2a.proto`
- Update specification documentation

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct