Add a server-side apply example

[thev1ndu]

What type of PR is this?

/kind documentation

What this PR does / why we need it:

The ServerSideApply utility in the util module has no accompanying example, even though server-side apply is a common operation. This adds ServerSideApplyExample, which:

• applies a ConfigMap with a field manager,
• re-applies an updated manifest with forceConflicts to show idempotent updates and field ownership,
• runs a dry-run apply that the server validates without persisting.

It follows the style of the existing examples and passes spotless:check.

Which issue(s) this PR fixes:

None

Does this PR introduce a user-facing change?

NONE

[kubernetes-prow]

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[brendandburns]

/lgtm
/approve

[kubernetes-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: brendandburns, thev1ndu

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