[Feat]: agent username field in agent card (extension)

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

In our systems, we plan to give each agent an identity that allows it to be referenced from any system. E.g. GitHub, Jira, Slack. To do so, it needs to be possible to mention the agent, e.g. "@agent-name do the thing", and therefore it needs a SSO login, just like a human.

Humans have usernames, so agents will too.

### Describe the solution you'd like

A new field in the Agent Card named "username".

### 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 Agent Card component in the codebase, likely in a frontend or shared model directory. Understand how agents are currently defined and where their data is stored. The change involves adding a 'username' field to the data model and updating the UI to display it. Check for existing tests related to agent creation or display to ensure the new field is properly integrated.

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