Support custom CRDs for ExtensionServer in Standalone Mode

[funkluk]

Description:
Extending Envoygateway functionality with Extension Server works in a Kubernetes environment. However, when trying to apply the example from the documentation in the standalone mode, error messages are logged during startup:

2025-10-03T13:24:31.595Z        ERROR   provider        file/file.go:100        failed to reload resources initially    {"runner": "provider", "error": "failed to load resources from file /tmp/envoy-gateway/config/gateway.yaml: no kind \"ListenerContextExample\" is registered for version \"example.extensions.io/v1alpha1\" in scheme \"pkg/runtime/scheme.go:110\""}

Despite the title of #5918 the support of the extension server in standalone mode is not complete.

Environment:

• Envoy Gateway v1.5.2

[daum3ns]

#5984 (comment)

[daum3ns]

hello all :)
I am working on this in this branch on my fork:
https://github.com/daum3ns/envoy-gateway/tree/support-custom-CRDs-for-ExtensionServer-in-Standalone-Mode

manual tests show its working with these changes.
I will try to add testcases for this...

also i was wondering during development if we might have a similar issue with ExtensionManagers backendResources, will try to figure that out... but maybe someone knows already , just looking at the code it feels like there is missing something: daum3ns@d41f12c

[arkodg]

thanks for picking this one up @daum3ns , should be easier to review and make suggestions once the PR is up

[github-actions]

This issue has been automatically marked as stale because it has not had activity in the last 30 days.