memory, config: reject connections when memory usage is high

[djshow832]

What problem does this PR solve?

Issue Number: close #1119

Problem Summary:
TiProxy may OOM when the connection count is too large

What is changed and how it works:

• Reject new connections when memory usage is too high
• Report unhealthy on the status port when the memory usage is too high

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.

- Reject connections and report unhealthy when the memory usage is too high

[ti-chi-bot]

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Details

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 71.42857% with 26 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@30d5b3f). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
pkg/manager/memory/memory.go	62.06%	15 Missing and 7 partials ⚠️
pkg/proxy/proxy.go	83.33%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1120   +/-   ##
=======================================
  Coverage        ?   67.17%           
=======================================
  Files           ?      144           
  Lines           ?    15158           
  Branches        ?        0           
=======================================
  Hits            ?    10183           
  Misses          ?     4276           
  Partials        ?      699           

Flag	Coverage Δ
unit	67.17% <71.42%> (?)

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[djshow832]

@codex review