event-exporter: fix OOM issue, conditionally enable WatchListClient for pod-owner-label

[zxqlxy]

WatchListClient should fix the OOM issue. Will default to WatchListClient later for watcher.

[erain]

Two blocking issues here:

1. event-exporter/.gitignore now ignores vendor/**. This repo commits event-exporter/vendor, so that pattern causes newly added vendored packages to stay ignored and never make it into the PR.

2. As submitted, the supported -mod=vendor build path is broken. event-exporter/Makefile and event-exporter/Dockerfile both build/test with -mod=vendor, but the PR adds dependencies/imports that are referenced in vendor/modules.txt without the corresponding source trees actually being present in event-exporter/vendor.

I verified this by running go test -mod=vendor ./... in event-exporter, which fails on this branch with missing packages such as k8s.io/client-go/metadata and sigs.k8s.io/structured-merge-diff/v6/value. After rerunning go mod vendor, the same command passes. Please regenerate and commit the full vendor tree and drop the vendor/** ignore.

[erain]

The vendor/** ignore is blocking the vendor refresh required by this change. Since this repo checks in event-exporter/vendor, this pattern causes newly added dependency directories to stay ignored and never make it into the PR.

[zxqlxy]

Must have added to make the file change clear when debugging, fixed now