…ache#12892)

ATS Configuration Reload with observability/tracing — Token model
Replace the fire-and-forget configuration reload mechanism with a new
token-based, observable reload framework. Every reload operation is now
assigned a unique token, tracked through a task tree, and queryable via
CLI or JSONRPC at any point after submission.

Core components introduced:

- ConfigRegistry: centralized singleton for config file registration,
  filename records, trigger records, and reload handlers. Replaces the
  scattered registration across AddConfigFilesHere.cc and individual
  modules.
- ReloadCoordinator: manages reload session lifecycle including token
  generation, concurrency control (--force to override), timeout
  detection, and rolling history.
- ConfigReloadTask: tracks a single reload as a tree of sub-tasks with
  per-handler status, timings, and logs.
- ConfigContext: lightweight context passed to handlers providing
  in_progress(), complete(), fail(), log(), supplied_yaml(), and
  add_dependent_ctx(). Safe no-op at startup when no reload is active.
- ConfigReloadProgress: periodic checker that detects stuck tasks and
  marks them as TIMEOUT.

New traffic_ctl commands:

- config reload [-m] [-t <token>] [-d @file] [--force]
- config status [-t <token>] [-c all]

  All commands support --format json for automation and CI pipelines.

New JSONRPC APIs:

- admin_config_reload: unified file-based or inline reload with token,
  force, and configs parameters.
- get_reload_config_status: query reload status by token or get the
  last N reloads.

Migrated config handlers to ConfigRegistry: ip_allow, cache_control,
cache_hosting, parent_proxy, split_dns, remap, logging,
ssl_client_coordinator (with sni.yaml and ssl_multicert.config as
dependencies), ssl_ticket_key, records, and pre-warm. Static configs
(storage, volume, plugin, socks, jsonrpc) registered as inventory-only.

Removed legacy ConfigUpdateHandler/ConfigUpdateContinuation from
ConfigProcessor.h. Removed AddConfigFilesHere.cc in favor of
per-module self-registration.

Fixed duplicate handler execution for configs with multiple trigger
records (e.g. ssl_client_coordinator) by deduplicating against the
ConfigReloadTask subtask tree.

Added RecFlushConfigUpdateCbs() to synchronously fire pending record
callbacks after rereadConfig(), ensuring all subtasks are registered
before the first status poll.

New configuration records:
- proxy.config.admin.reload.timeout (default: 1h)
- proxy.config.admin.reload.check_interval (default: 2s)

Backward compatible: existing `traffic_ctl config reload` works as
before; internally it now uses the new framework with automatic token
assignment and tracking.

(cherry picked from commit 5bab268)

Conflicts:
	include/tscore/ArgParser.h
	src/iocore/cache/P_CacheHosting.h
	src/iocore/hostdb/CMakeLists.txt
	src/proxy/ReverseProxy.cc
	src/records/CMakeLists.txt
	src/tscore/ArgParser.cc