[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