feat(cloudflare): support custom hostname Origin SNI overrides

[mrozentsvayg]

What does it do ?

Extends the cloudflare-custom-hostname annotation to support Origin SNI
overrides using the format <customHostname>=<customOriginSNI>.

• No = suffix: SNI defaults to the origin server (existing behaviour, unchanged)
• Trailing = with no value: SNI is set to the request Host header
• =<value>: SNI is set to the specified hostname

The custom_origin_sni field is only sent to the Cloudflare API when
explicitly overridden (i.e. when SNI differs from the origin server),
since the field requires a Cloudflare account entitlement.

Motivation

Cloudflare for SaaS supports routing custom hostname traffic to different
backends based on SNI. Without SNI control, all custom hostnames must share
the same TLS routing, which is too restrictive for multi-tenant setups where
different customers need traffic routed to different origins at the TLS layer.

For example, when using Envoy Gateway in Merged Gateways deployment mode,
each custom hostname needs its own SNI to correctly route TLS traffic to the
right backend — something that wasn't possible before without manual
Cloudflare configuration.

More

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

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[k8s-ci-robot]

Hi @mrozentsvayg. Thanks for your PR.

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[ivankatliarchuk]

This PR is more about traffic routing than DNS management, so very high chances it may be out of scope. It would be worth double-checking with the owners on Slack to see what they think.

[ivankatliarchuk]

Similar one #6085

[AndrewCharlesHay]

Here is a RFC for a repo that can house this kind of stuff