Add AI plugin

[npolshakova]

Description

Adds Upstream and RoutePolicy plugin implementations. Requires #10493 and #10495 to merge first.

API changes

NONE (All API changes are in #10493)

Code changes

• Adds AI Upstream translation to upstream plugin
• Adds RoutePolicy translation to route policy plugin
• Introduces new ApplyBackend() which will apply route/typed filter config to backend regardless if policy is attached.

CI changes

NONE

Docs changes

Tracked separately in #10631

Context

See EP: #10495

Interesting decisions

• For the initial scope, limited support to extensionRef filter reference (no targetRef support) to simplify the assertion the backendRef the route policy is being attached to is an AI Upstream.

Testing steps

• Adds new ggv2setup_tests for passthrough, inline token, and secretRef (with different providers). These test cases also cover configuring route policies with defaults, prompt enrichment, prompt guard and streaming.

Notes for reviewers

TODO: add steps for reproducing locally

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works

[andy-fong]

It will be easier to read if this is set to merge to npolshakova:port-ai-apis-to-kgateway

[npolshakova]

It will be easier to read if this is set to merge to npolshakova:port-ai-apis-to-kgateway

Yep, rebased but the PRs are both in my fork. Here's the cleaner diff: npolshakova#1