Description

This PR adds an opt-in persistent backing for a cluster's cross-priority host map. A dynamic modules cluster can call the new envoy_dynamic_module_callback_cluster_use_persistent_host_map callback during cluster init
to replace the default flat copy-on-write map with an immer persistent map, turning each membership update from an O(N) copy into an O(delta) update.

The map is published through a new HostLookupTable interface so the backing is swappable without changing consumers. The flat map stays the default and membership behavior is identical under either backing.

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Commit Message: dynamic_modules: per-cluster persistent cross-priority host map via cluster ABI
Risk Level: Low
Testing: Added Tests
Docs Changes: Added
Release Notes: Added

Request variables

Key	Value
ref	e91b2d1
sha	472904b
pr	45842
base-sha	467ee8f
actor	@agrawroh
message	dynamic_modules: per-cluster persistent cross-priority host map via cluster ABI...
started	1782501365.262266
target-branch	main
trusted	false

Container image/s (as used in this CI run)

Envoy version (as used in this CI run)