backend, net: support connection attrs in COM_CHANGE_USER

[djshow832]

What problem does this PR solve?

Issue Number: close #355

Problem Summary:

• Connection attributes in COM_CHANGE_USER are not saved, so the connection attrs may be wrong after session migration.

What is changed and how it works:

• Parse connection attrs in COM_CHANGE_USER, passes it to TiDB (previously not), and sends it to TiDB in the second handshake.
• After COM_CHANGE_USER, migrate the session if needed. Previously it didn't check the redirection signal after COM_CHANGE_USER.
• Add comments for why we need another switch-auth request for COM_CHANGE_USER. The reason is here: create and switch user failed on tiproxy #127 (comment)

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Integration test: testBug19354014 in JDBC test.

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

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-09-12 11:10:49.956131373 +0000 UTC m=+354031.880687766: ☑️ agreed by xhebox.