wip: dynamic_modules: let runtime cluster hosts carry logical hostnames

[dio]

Commit Message:

dynamic_modules: let runtime cluster hosts carry logical hostnames

Additional Description:

Add an opt-in C ABI callback and Rust SDK methods that let dynamic-module clusters set a logical hostname separately from the connection address. This enables existing features such as auto_host_sni and automatic SAN validation.

The existing callback and its behavior remain unchanged. Effective-SNI-scoped TLS session caching was handled in #45982; distinct logical hosts sharing one address can be considered separately.

AI assistance was used to help implement this change. I reviewed and understand the submitted code and take responsibility for it.

Risk Level:

Medium. This adds an opt-in extension ABI and SDK surface.

API Considerations:

Adds an opt-in C ABI callback and Rust SDK methods. Existing callbacks and behavior remain unchanged.

Testing:

• //test/extensions/clusters/dynamic_modules:cluster_test
• Envoy CI

Docs Changes:

Added C ABI and Rust SDK API documentation.

Release Notes:

Added a dynamic-modules release note.

Fixes #46387
Refs #45962

[repokitteh-read-only]

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🐱

Caused by: #46388 was opened by dio.

see: more, trace.

[dio]

Reviewer note on the Go reproducer and SDK boundary:

The default scenario in https://github.com/dio/auto-sni-choose-host is a working Go-based e2e demonstration of this PR. The module supplies a concrete address and logical hostname for each runtime host, retains the returned host handles, and returns the selected handle from ChooseHost based on x-target-host. Envoy then uses the selected host hostname with one shared UpstreamTlsContext configured with auto_host_sni and auto_sni_san_validation. The README provides the runnable commands (make run, make request-example, and make request-iana) and downloadable patched Envoy binaries.

The demo imports Envoy SDK package paths, but its go.mod temporarily replaces the module with https://github.com/dio/envoy/tree/dio/dynamic-module-go-cluster-sdk-hostnames. Upstream currently has no Go dynamic-module cluster API, so that branch supplies the Go lifecycle/ABI bridge; the bridge is compiled into the Go shared module, while the downloaded Envoy binary contains only this PR’s C++ ABI change. The Rust changes in this PR are smaller because they extend the existing upstream Rust cluster SDK.

I have deliberately kept the greenfield Go cluster SDK out of this PR. If an upstream Go cluster SDK is desirable, I can propose the prepared implementation as a separate follow-up; otherwise the C ABI added here remains language-neutral and Go modules can provide their own bridge. Reviewer guidance on whether that separate Go SDK surface is wanted would be helpful.

The demo also documents an optional shared-address scenario, but that uses a separate stacked Envoy patch and is not evidence or scope for this PR.

[dio]

/retest