Endpoint Picker Support

[Xunzhuo]

Description:

This issue tracks the follow up tasks for endpoint picker support

• Docs: Add initial Design Docs
• Functionality: Support InferencePool in AIGatewayRoute
• Functionality: Support InferencePool in HTTPRoute
• Docs: Update Design Docs
• Docs: Add User-Guide Docs
• Docs: Update Blog
• Testing: Support Upstream Conformance Test

[mathetake]

Functionality: Envoy AI GW EPP Implementation

let's remove this from the TODOs as we haven't decided anything on that matter, even whether or not we do have our impl in the first place. That should be a huge topic we should discuss separately, including at the community meeting.

Functionality: Support Host Override LbPolicy and fallback

also this is not a blocker for us to have a support this feature, not a must. let's remove it