feat: add --txt-prefix-override flag to support apex domains safely

[u-kai]

What does it do ?

This PR introduces a new flag, --txt-prefix-override, as a safe and flexible workaround
to support apex domains when using the TXT registry.

The flag allows users to override the TXT record prefix per domain by specifying
one or more mappings in the following form:

--txt-prefix-override=domain=txt-prefix

Multiple overrides can be provided, and each mapping applies only to the explicitly
specified domain.

Motivation

In #5864, we discussed limitations and edge cases around managing apex domains with the
TXT registry.

While a flag such as --apex-domains was considered, it was intentionally avoided for
the following reasons:

• A flag named --apex-domains could be misunderstood as applying special behavior to
  all apex domains automatically.
• The underlying problem is not limited to apex domains; it is more generally about
  domain-specific TXT prefix customization.
• Introducing a domain-scoped override mechanism keeps the behavior explicit and avoids
  surprising users.

By using --txt-prefix-override, users must opt in per domain, making the behavior
clear and predictable.

More

• Yes, this PR title follows Conventional Commits
• Yes, I added unit tests
• Yes, I updated end user documentation accordingly

[k8s-ci-robot]

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[k8s-ci-robot]

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