router: add port-based routing groups

[YangKeao]

What problem does this PR solve?

Issue Number: close #1100

• 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

Add port-based routing to the score-based router.

This PR introduces:

• routing-rule = "port"
• routing by the SQL listener port
• cluster-scoped port groups
• cross-cluster port-conflict detection and blocking

What is changed and how it works:

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

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 85.08772% with 17 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@17250b1). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
pkg/balance/router/port_conflict_detector.go	67.74%	7 Missing and 3 partials ⚠️
pkg/balance/router/router_score.go	94.44%	3 Missing and 1 partial ⚠️
pkg/proxy/backend/backend_conn_mgr.go	0.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1106   +/-   ##
=======================================
  Coverage        ?   67.43%           
=======================================
  Files           ?      146           
  Lines           ?    15361           
  Branches        ?        0           
=======================================
  Hits            ?    10359           
  Misses          ?     4303           
  Partials        ?      699           

Flag	Coverage Δ
unit	67.43% <85.08%> (?)

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[YangKeao]

/test all

[YangKeao]

/test all