backend, net: Remind the user when TiDB sets wrong proxy-protocol

[djshow832]

What problem does this PR solve?

Issue Number: close #360

Problem Summary:
Users are very likely to misconfigure proxy-protocol. And they do, they are hard to find the reason according to the error message.

What is changed and how it works:

• Remind the user when TiDB and TiProxy proxy-protocol mismatch.
• Move zap field proxy-protocol to the place where the connection is created.
• Refine other error messages.

Check List

Tests

• Unit test

• Integration test

• Manual test (add detailed scripts or steps below)

• No code

• Enable TiProxy PPV2, disable TiDB PPV2, enable TLS

• Enable TiProxy PPV2, disable TiDB PPV2, disable TLS

• Disable TiProxy PPV2, enable TiDB PPV2, enable TLS

• Disable TiProxy PPV2, enable TiDB PPV2, disable TLS

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]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: xhebox

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [xhebox]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-09-07 13:09:24.216659863 +0000 UTC m=+2623728.765675845: ☑️ agreed by xhebox.