original_dst: add batched host updates

[jronak]

Commit Message: Add batched host updates in original_dst cluster

Additional Description: Adds batching support for host updates in original_dst clusters to address performance bottlenecks on the main thread. With high fan-out clients with large numbers of active connections (e.g. 60k) and high unique host-port connection rates (e.g. 3k/s), the current per-connection addHost callbacks create significant pressure on the main thread. Each callback triggers a host map copy and priority set update, which is CPU heavy and impacts critical operations like metrics collection + health checks on the main thread.

The new batch_update_interval configuration option allows consolidating these updates into periodic batches, significantly reducing main thread overhead. This has been production-tested for 6+ months with positive results.

Risk Level: Low
Testing: Unit tests
Docs Changes: proto docs
Release Notes: None
Platform Specific Features: None

[repokitteh-read-only]

CC @envoyproxy/api-shepherds: Your approval is needed for changes made to (api/envoy/|docs/root/api-docs/).
envoyproxy/api-shepherds assignee is @wbpcode
CC @envoyproxy/api-watchers: FYI only for changes made to (api/envoy/|docs/root/api-docs/).

🐱

Caused by: #38769 was opened by jronak.

see: more, trace.

[wbpcode]

/lgtm api

[wbpcode]

could you check the CI first, thanks?

[nezdolik]

friendly ping @jronak, looks like CI needs fixing

[phlax]

@jronak needs main merge

[phlax]

/wait

[jronak]

@phlax ack, I'll rebase the PR soon. Apologies, I was unavailable for a week.

[github-actions]

This pull request has been automatically marked as stale because it has not had activity in the last 30 days. It will be closed in 7 days if no further activity occurs. Please feel free to give a status update now, ping for review, or re-open when it's ready. Thank you for your contributions!

[github-actions]

This pull request has been automatically closed because it has not had activity in the last 37 days. Please feel free to give a status update now, ping for review, or re-open when it's ready. Thank you for your contributions!