[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

Find the a2a.proto file, locate the AgentCard message definition, and add the optional string field with the correct field number. Update any related specification documentation. Verify the change by checking that the protocol buffer compiles correctly.

Scritto dal modello di indicizzazione a partire dal testo della issue.