*: support proxy protocol on reconnection

[xhebox]

Signed-off-by: xhe xw897002528@gmail.com

What problem does this PR solve?

Issue Number: ref #14

Problem Summary: Support proxy-protocol for redirection.

What is changed and how it works:

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
  tiup playground --db.config xx where xx is a file of following contents:

[security]
auto-tls = true
[proxy-protocol]
networks = "*"
timeout = 10

Enable proxy protocol on tiproxy and start.
Connect to tiproxy by mycli -u root -P 6000.
curl -X "POST" -L http://127.0.0.1:3080/api/debug/redirect will see redirecti connection succeeds in tiproxy log.

• No code

Notable changes

• Has configuration change
• Has HTTP API interfaces change (Don't forget to add the declarative for API)
• Has tiproxyctl change
• Other user behavior changes

Release note

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None