backendcluster: route PD and backend traffic through cluster DNS

[YangKeao]

What problem does this PR solve?

Issue Number: close #1101

• 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:

Add cluster-scoped DNS routing for backend-cluster traffic.

This PR makes each backend cluster use its configured ns-servers for:

• PD / etcd traffic
• infosync-related topology lookups
• backend HTTP / MySQL traffic created from the cluster runtime

If ns-servers is empty, the default system resolver is still used.

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.
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[codecov-commenter]

Codecov Report

❌ Patch coverage is 67.04438% with 349 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@ca24d03). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
pkg/testkit/dns_server.go	0.00%	95 Missing ⚠️
pkg/manager/backendcluster/manager.go	65.50%	72 Missing and 17 partials ⚠️
pkg/manager/backendcluster/metrics_querier.go	65.51%	27 Missing and 3 partials ⚠️
pkg/util/etcd/etcd.go	60.00%	19 Missing and 9 partials ⚠️
lib/config/proxy.go	83.33%	15 Missing and 6 partials ⚠️
pkg/proxy/proxy.go	15.38%	9 Missing and 2 partials ⚠️
pkg/balance/router/port_conflict_detector.go	67.74%	7 Missing and 3 partials ⚠️
pkg/util/netutil/dns.go	85.07%	7 Missing and 3 partials ⚠️
pkg/balance/observer/health_check.go	74.19%	5 Missing and 3 partials ⚠️
pkg/proxy/backend/backend_conn_mgr.go	66.66%	6 Missing and 2 partials ⚠️
... and 9 more

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1107   +/-   ##
=======================================
  Coverage        ?   66.84%           
=======================================
  Files           ?      148           
  Lines           ?    15564           
  Branches        ?        0           
=======================================
  Hits            ?    10403           
  Misses          ?     4447           
  Partials        ?      714           

Flag	Coverage Δ
unit	66.84% <67.04%> (?)

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[YangKeao]

/test all

[YangKeao]

/test all