Reuse controller manager

[dmitri-d]

Description

• Motivation: Controller manager is currently instantiated in ControllerBuilder, with all controllers added to it as part of NewControllerBuilder call, and without any way to register additional controllers without changing ControllerBuilder implementation.
  It should be possible to update kgateway setup/initialization to support creating additional controllers without creating additional instances of controller manager.
• What changed: Extended setup api to support updating the default controller manager configuration, such as registering additional controllers.

Change Type

/kind cleanup

Changelog

Added support for updating the default controller manager configuration with custom changes on startup.

Additional Notes

[Copilot]

Pull Request Overview

Extends the kgateway startup API to reuse a single controller manager instance and allow external registration of additional controllers and manager options.

• Introduced functional options for REST config, controller-manager options, and extra manager configuration.
• Refactored setup flow: replaced multiple StartKgateway variants with a unified Start→BuildKgatewayWithConfig→mgr.Start sequence.
• Updated tests and examples to use the new manager-based setup API.

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
test/envtestutil/util.go	Switched test harness to instantiate a controller-runtime Manager and call the new BuildKgateway API.
pkg/setup/setup.go	Added RestConfig, CtrlMgrOptions, and ExtraManagerConfig to Options, removed core.AddToScheme invocation.
internal/kgateway/setup/setup.go	Added functional option builders (WithRestConfig, WithControllerManagerOptions, WithExtraManagerConfig), removed old XDS-specific entrypoints, unified Start.
internal/kgateway/controller/start.go	Updated StartConfig to accept an injected manager.Manager, removed scheme-extension callback via AddToScheme, wired through cfg.Manager.
examples/plugin/main.go	Replaced direct StartKgateway call with setup.New(...).Start(...) to demonstrate the new API.

Comments suppressed due to low confidence (1)

pkg/setup/setup.go:28

• Add a GoDoc comment for RestConfig (and the other new fields) in Options to clarify their purpose.

	RestConfig               *rest.Config

[Copilot]

[nitpick] In a test goroutine, use t.Fatalf or require.NoError instead of panic so the failure is reported by the test harness.

Suggested change

	panic("failed to create a manager")
	t.Fatalf("failed to create a manager: %v", err)

[timflannagan]

@dmitri-d I think we can skip release note here?