backendcluster: add cluster manager and cluster-scoped topology runtime

[YangKeao]

What problem does this PR solve?

Issue Number: close #1098

• docs: add cluster-scoped multi-cluster runtime design #1102
• config: add backend cluster schema and multi-port listener config #1103
• backendcluster: add cluster manager and cluster-scoped topology runtime #1104 <-
• metricsreader: move cluster-scoped metrics lifecycle into backend clusters #1105
• router: add port-based routing groups #1106
• backendcluster: route PD and backend traffic through cluster DNS #1107

What is changed and how it works:

Introduce a backend-cluster manager that owns cluster-scoped runtime instances.

This PR adds:

• a manager for configured backend clusters
• one runtime per backend cluster
• cluster-scoped etcd / infosync / shared clients
• topology aggregation across clusters
• dynamic add / update / remove handling when backend-cluster config changes

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Notable changes

• Has configuration change
• Has HTTP API interfaces change
• Has tiproxyctl change
• Other user behavior changes

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[ti-chi-bot]

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Details

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 65.88235% with 116 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@29264bd). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
pkg/manager/backendcluster/manager.go	60.37%	66 Missing and 18 partials ⚠️
pkg/server/server.go	58.82%	6 Missing and 1 partial ⚠️
pkg/balance/observer/health_check.go	44.44%	4 Missing and 1 partial ⚠️
pkg/proxy/backend/backend_conn_mgr.go	54.54%	4 Missing and 1 partial ⚠️
pkg/balance/router/router.go	55.55%	4 Missing ⚠️
pkg/balance/router/router_static.go	0.00%	4 Missing ⚠️
pkg/manager/namespace/manager.go	0.00%	4 Missing ⚠️
pkg/balance/router/group.go	81.25%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1104   +/-   ##
=======================================
  Coverage        ?   67.15%           
=======================================
  Files           ?      143           
  Lines           ?    15067           
  Branches        ?        0           
=======================================
  Hits            ?    10118           
  Misses          ?     4253           
  Partials        ?      696           

Flag	Coverage Δ
unit	67.15% <65.88%> (?)

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[YangKeao]

/test all

[YangKeao]

TestLocalDirWatcher is not stable. Let me try to stablize it.

/retest

[YangKeao]

/test all