delete forward sni, cluster rewrite, and sni verifier filters

[kyessenov]

No longer used in Istio, and should be rewritten as Wasm if still needed.

Deletes some unused bazel dependencies.

Issue: envoyproxy/envoy#29681

[lei-tang]

LGTM as Istio no longer uses these three filters. Beside Istio, are there other projects/products that use Istio proxy and these filters? Consider providing an advance notice (e.g., in Istio release notes, or on istio.io) about removing these filters to avoid breaking their users.

[kyessenov]

The only way to use those is via EnvoyFilter, and there's no integration test, no doc, and no reference in the istio code base to them. We'll post a release note, but I think it's OK to try to remove them for now, and if there's a strong complaint we can get them back.

[frankbu]

@kyessenov Removing forward_downstream_sni will break IBM (and presumably other) production clusters that are still using the old egress wildcard pattern. We are going to switch to an alternative approach, but I think advance notice of this change is needed, before deleting it, because it was previously documented.

[kyessenov]

@frankbu Ack, I checked with @howardjohn first, but didn't see EnvoyFilter in the docs. I think we can get envoyproxy/envoy#29844 from upstream merged by the release of 1.20, and it's going to be functionally equivalent. If not, we'll bring it back. I hope there's at least some testing somewhere in istio.