Support k8s Gateway API and ingress path prefix

[davidshen84]

Currently, the open-webui chart uses the outdated Ingress resource to define the ingress control and http route.

It should move to the new Kubernetes Gateway API and provide URL prefix rewrite.

[0xThresh]

Hey @davidshen84, I started looking at this one and agree with the change, just haven't had the time to implement it yet. PRs are welcome, otherwise this is one of the first issues I'll work on when I get the chance.

[davidshen84]

The challenge of this is the hardcoded resource URLs, like https://github.com/open-webui/open-webui/blob/main/src/app.html#L5.

There's no easy way to insert the prefix to them.

For backend apps, they are behind the ingress gateway router, and the URL rewrite can handle the prefix easily.