Enable envoy token rate limiting by configuration

[kerthcet]

What would you like to be added:

Envoy AI gateway supports token-based rate limiting, see https://aigateway.envoyproxy.io/docs/getting-started/installation#configuring-envoy-gateway, make this configurable with helm chart, also provides documentations.

Why is this needed:

Envoy uses redis for rate limiting, which will also be used by metrics aggregator as the default backend storage.

Completion requirements:

This enhancement requires the following artifacts:

• Design doc
• API change
• Docs update

The artifacts should be linked in subsequent comments.

[kerthcet]

/help

[googs1025]

/assign

[googs1025]

I will handle this in weekend

[kerthcet]

Ping on this, the ai gateway routing is depend on this.

[googs1025]

Update: The update we depend on seems to be in the master branch of Envoy AI gateway. Should we wait until Envoy AI gateway releases a stable version before developing this part? 🤔

[kerthcet]

I think this is already released in v0.1.0, see https://github.com/envoyproxy/ai-gateway/releases/tag/v0.1.0.