[QueryThrottler]Replace polling with topo server watch for config updates

[siddharth16396]

This PR is clone of #18905 which i messed up pretty badly while rebasing for DCO commits 😢

Description

This PR Replaces periodic config polling (60s intervals) with event-driven topology server watches for QueryThrottler configuration updates. Instead of continuously reading config files on a timer, the throttler now subscribes to SrvKeyspace changes and reacts immediately when configs are updated in the topo server.

Changes Made

Removed the polling model:

• Deleted ConfigLoader interface and FileBasedConfigLoader implementation.
• Removed 60-second refresh loop from QueryThrottler.
• Eliminated file-based config loading entirely.

Added event-driven topo watch:

• Introduced startSrvKeyspaceWatch() that uses srvtopo.WatchSrvKeyspace() for real-time updates
• Added HandleConfigUpdate() callback for processing config changes
• Implemented smart deduplication to avoid redundant strategy reloads
• Added resilient error handling (continues watching on transient errors, stops only on permanent ones like NoNode)

New proto definitions:

• Created querythrottler.proto with Config, ThrottlingStrategy, and nested rule structures
• Added incoming_query_throttler_config field to SrvKeyspace message in topodata.proto
• Implemented ConfigFromProto() conversion between protobuf and internal config representation

Architecture improvements:

• Config now lives in topology server (centralized, versioned, observable)
• Tablets get instant updates when operators change throttling rules
• Initial config loaded synchronously during InitDBConfig() to ensure correct state on startup/restart
• Watch continues indefinitely with auto-retry for transient failures

How it works

1. Startup: InitDBConfig() sets the keyspace and immediately fetches initial config via GetSrvKeyspace()
2. Watch: Background goroutine subscribes to WatchSrvKeyspace() for the cell+keyspace
3. Updates: When config changes in topo, HandleConfigUpdate() fires, deduplicates, and hot-swaps the strategy
4. Resilience: Network blips are retried automatically; only fatal errors (context canceled, keyspace deleted) stop the watch

Benefits achieved:

• Faster propagation: Config changes apply immediately instead of waiting up to 60s
• Less load: No more periodic file reads from every tablet
• Better ops: Centralized config in topo server (can use vtctldclient to update)
• Consistency: All tablets see changes at roughly the same time
• Debugging: Topo changes are auditable (vs. scattered config files)

Related Issue(s)

• Question: v23.0.0 Error loading config: open /config/throttler-config.json: no such file or directory #18869

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

[vitess-bot]

Review Checklist

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
• Ensure there is a link to an issue (except for internal cleanup and flaky test fixes), new features should have an RFC that documents use cases and test cases.

Tests

• Bug fixes should have at least one unit or end-to-end test, enhancement and new features should have a sufficient number of tests.

Documentation

• Apply the release notes (needs details) label if users need to know about this change.
• New features should be documented.
• There should be some code comments as to why things are implemented the way they are.
• There should be a comment at the top of each new or modified test to explain what the test does.

New flags

• Is this flag really necessary?
• Flag names must be clear and intuitive, use dashes (-), and have a clear help text.

If a workflow is added or modified:

• Each item in Jobs should be named in order to mark it as required.
• If the workflow needs to be marked as required, the maintainer team must be notified.

Backward compatibility

• Protobuf changes should be wire-compatible.
• Changes to _vt tables and RPCs need to be backward compatible.
• RPC changes should be compatible with vitess-operator
• If a flag is removed, then it should also be removed from vitess-operator and arewefastyet, if used there.
• vtctl command output order should be stable and awk-able.