Manage the ai gateway resources automatically

[kerthcet]

What would you like to be added:

From the ai gateway example, we'll create the gateway resources manually, this is just because envoy ai gateway is not that mature right now, for example, the health probe support: envoyproxy/ai-gateway#587.

Once the issue is fixed, we should pipeline this inside the controller. Also, we should consider how to set the configurations, like the weights.

Why is this needed:

Easy to use with llmaz.

Completion requirements:

This enhancement requires the following artifacts:

• Design doc
• API change
• Docs update

The artifacts should be linked in subsequent comments.

[kerthcet]

cc @pacoxu