Introduce AG-UI support into Agent Gateway

[g3force]

The AG-UI protocol is an open, lightweight, event-based protocol that standardizes how AI agents connect to user-facing applications.

Currently, our agents only speak A2A (officially), but the agent frameworks support AG-UI already.

There are two ways to proceed:

1. Just add a proxy to the agent-gateway and expose AG-UI for each agent, thus extending Agent CRD
2. Create a mapper in the agent-gateway to map from AG-UI to A2A

Next step: Analyze the protocols and check if it is actually feasible to implement such a mapping. Probably, first option is the easier way to go for now, until there are agent gateways that implement a mapping.

[g3force]

AG-UI docs point to this: https://docs.copilotkit.ai/learn/a2a-protocol