Add agentgateway as an AI Inference Gateway

The [Networking for AI Inference Model Serving](https://github.com/GoogleCloudPlatform/cloud-networking-solutions/tree/main/reference-architectures/networking-for-ai-inference) reference architecture supports GKE Inference Gateway and Self Managed Options. [Agentgateway](https://agentgateway.dev/) is an open source AI gateway that can support both options or can be added as a third option.

Start by reading the reference architecture in reference-architectures/networking-for-ai-inference and reviewing how the GKE Inference Gateway and self-managed options are represented. Determine whether agentgateway should support those options or appear as a third option; done means the architecture clearly documents agentgateway and its supported deployment path.

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