feat: add Server-Side Apply for object synchronization

[blut]

on-behalf-of: @eon-se opensource@eon.com

Summary

Introduce an opt-in --enable-server-side-apply flag that switches the
sync controller from client-side merge patches (backed by a last-known-
state secret) to Kubernetes Server-Side Apply with a stable field
manager ("api-syncagent/").

SSA preserves fields owned by other controllers, notably
Crossplane's spec.resourceRef on downstream claims. This prevents
the full-update fallback from clobbering those fields and creating
duplicate composite resources.

Key changes:

• New --enable-server-side-apply flag (default: false)
• applyServerSide() in objectSyncer handles create-or-update in a
  single Apply round-trip with ForceOwnership to recover from prior
  client-side field managers

What Type of PR Is This?

/kind feature

Related Issue(s)

Fixes #

Release Notes

NONE

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[kcp-ci-bot]

Hi @blut. Thanks for your PR.

I'm waiting for a kcp-dev member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[ntnn]

/ok-to-test

[kcp-ci-bot]

@blut: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-api-syncagent-lint	3470854	link	true	/test pull-api-syncagent-lint

Full PR test history

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[mjudeikis]

@blut you PR needs changing as we did some upstream multicluster-runtimen changes

[xrstf]

If SSA now preserves ownership of fields, does this influence how we keep track of previous states to form proper patches?

[blut]

If SSA now preserves ownership of fields, does this influence how we keep track of previous states to form proper patches?

I think it could, but I didn't bother changing any implementation, to avoid breaking behaviour. That's the reason for the flag as well. This was to get a first discussion started.

Based on my initial thoughts: With SSA we could remove the stored state and rely on SSA to compute the correct merged state. There needs to be some thought about mutating webhooks, to avoid reconcile loops.