EndpointSelector resource for creating EndpointSlices without Service

### Enhancement Description Today, any user/controller that wants a set of `EndpointSlices` for a set of pods matching a label selector, they have 2 options: 1. Create a `Service` 2. Create and update the `EndpointSlice` manually as pods matching that selector spin up and down The latter tends to be avoided by controllers when possible since there is a real impact to scalability when you update endpoints that rapidly across a large cluster. The Kubernetes control plane is already capable of doing this for `Service` resources, so it stands to reason that this functionality become available to arbitrary consumers as well. Thus, many controllers end up settling for option 1, creating a Service whenever they need `EndpointSlices`. However, this is not always desirable because `Service` comes with a bunch of other _stuff_ that you may not want (e.g. a DNS hostname, a frontendIP, etc.). One recent example of this complexity is the `InferencePool` resource in the Gateway API inference extension. That resource needs to describe a pool of inference endpoints in the cluster; however, using `Service` to do so would inevitably result in users calling the InferencePool using its hostname/cluster VIP, resulting in kube-proxy load balancing that negates all of the performance benefits of the endpoint picker architecture. And due to only have the aforementioned two options for endpont generation available to them, GAIE implementations end up bearing the complexity of making custom load balancing work. This KEP is meant to serve as the root of the parallel Gateway API [GEP](https://github.com/kubernetes-sigs/gateway-api/pull/4731) describing this functionality. The idea is that that GEP would be permanently experimental in order to give implementations (and ecosystem subprojects like Kube-Agentic-Networking and WG AI Gateway) the ability to prototype and get feedback before this KEP becomes GA. This would also give us the opportunity to address some longstanding feature requests like kubernetes/kubernetes#62795 and kubernetes/kubernetes#48528. /ccing some interested folks: @mikemorris @robscott @LiorLieberman @bowei @aojea @danwinship @youngnick @kflynn @rikatz - One-line enhancement description (can be used as a release note): Introduce a new resource to allow creating EndpointSlices from selectors without having to create a `Service` resource - Kubernetes Enhancement Proposal: TBD - Discussion Link: Here's the [recording](https://youtu.be/OmD_fKasCNA?si=xpcOhcPgUd7_mbQw&t=1083) of me presenting this [slide deck](https://docs.google.com/presentation/d/1h_2WYyvIbyyCIMN61FInAfFtaJk_TrYpDXKoHXnUfy8/edit?slide=id.g3defe1ff0d5_0_12#slide=id.g3defe1ff0d5_0_12) on the functionality described in the KEP. - PRs by stage and milestone: - [ ] Alpha - v1.38 - [ ] KEP (`k/enhancements`) update PR(s): - [ ] Code (`k/k`) update PR(s): - [ ] Docs (`k/website`) update PR(s): <!-- Uncomment these as you prepare the enhancement for the next stage - [ ] Beta - v1.xx - [ ] KEP (`k/enhancements`) update PR(s): - [ ] Code (`k/k`) update PR(s): - [ ] Docs (`k/website`) update(s): - [ ] Stable - v1.xx - [ ] KEP (`k/enhancements`) update PR(s): - [ ] Code (`k/k`) update PR(s): - [ ] Docs (`k/website`) update(s): -->