Make ShowManagedFields public so printers users can tweak the value

[soltysh]

What type of PR is this?

/kind cleanup
/sig cli

What this PR does / why we need it:

In #96878 we introduced flags allowing to hide .metadata.managedFields but we didn't give cli-runtime consumers option to
modify it, just like all the remaining printers have. This PR changes that field to be public and thus modifiable.

Special notes for your reviewer:

/assign @deads2k

Does this PR introduce a user-facing change?

NONE

[k8s-ci-robot]

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[deads2k]

/lgtm

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