Native Envoy Gateway features for enabling client side / egress Gen AI use cases

*Description*:
>Describe the desired behavior, what scenario it enables and how it
would be used.

https://github.com/envoyproxy/ai-gateway is focused on building solutions to handle request traffic from application clients to Generative AI services on top of Envoy Gateway.

Some of the feature requirements may not be Gen AI specific and can be represented in the Gateway API or Gateway API extensions within Envoy Gateway in a neutral way and perused for other domains and use cases.

Creating this top level issue to track these features.

- [ ] Connecting to an AWS service https://github.com/envoyproxy/gateway/issues/688
- [x] Appending an API token #4757
- [x] #4756
- [ ] #4758

[optional *Relevant Links*:]
>Any extra documentation required to understand the issue.