feat(standalone): use typed labels.Selector for endpoint selector

[shichaooutlook]

Replace the string-based --endpoint-selector flag with Kubernetes-native labels.Selector to support set-based label expressions (e.g., env in (prod,staging),tier!=frontend, key,!deprecated) in
standalone mode.
Motivation

 The current --endpoint-selector only supports simple key=value pairs via labels.ConvertSelectorToLabelsMap. This is too restrictive — operators running llm-d outside Kubernetes may need set-based
 selectors to match pods by environment, tier, or exclusion patterns. The upstream code already had a TODO comment: // TODO: k8s.Selector, pflag.StringSlice? — this PR implements that TODO.
 Changes

 - Options.EndpointSelector: type changed from string to labels.Selector
 - Options.Complete(): parses the flag string via labels.Parse() and stores the result
 - EndpointPool.Selector: type changed from map[string]string to labels.Selector
 - Datastore.PoolLabelsMatch: simplified — uses native Selector.Matches() inline instead of constructing a temporary selector
 - Datastore.podResyncAll: passes labels.Selector directly to K8s List API
 - Datastore.selectorEqual: new nil-safe helper for labels.Selector comparison
 - InferencePoolToEndpointPool: uses labels.SelectorFromSet() instead of raw map
 - All callers and tests updated for the new type
 - CLI flag help text updated to document set-based syntax
 Test Plan

 - [x] go test ./pkg/epp/datastore/... — PASS
 - [x] go test ./pkg/epp/controller/... — PASS
 - [x] go test ./pkg/epp/server/... — PASS
 Notes for Reviewers

 - EndpointSelector is now nil (typed nil) when not set, replacing the previous empty string ""
 - All checks now use == nil / != nil instead of == "" / != ""
 - labels.Selector is an interface, not comparable with == — selectorEqual helper and cmp.Comparer handle this
 - Backward compatible at the CLI level: all previously valid key=value selectors remain valid

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