[Feat]: Unique identifier for an agent in Agent Card

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

Currently Agent Card does not have a unique identifier attribute for an agent. `agent_card.name` is not guaranteed unique and is prone to branding changes.

### Describe the solution you'd like

- Add a new`id` attribute to the agent card which must be a randomly generated UUID.
- This `id` must remain same for all versions of an agent.
- Mark the `id` as optional for backward compatibility.
- Clients can reliably use `id` as the identifier if set in the agent card.

### 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