backend: fix using password in the error message is wrong

[djshow832]

What problem does this PR solve?

Issue Number: close #332

Problem Summary:
When TiProxy handshakes with TiDB, it sends auth_plugin=unknown_auth_plugin and auth_data=nil. TiDB sees that auth_data=nil and then sets using password to NO.

What is changed and how it works:
Pass authData to TiDB for the first handshake.

Check List

Tests

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

mysql -h127.1 -udummy -P6001 -pdummy
mysql: [Warning] Using a password on the command line interface can be insecure.
ERROR 1045 (28000): Access denied for user 'dummy'@'127.0.0.1' (using password: YES)

mysql -h127.1 -udummy -P6001
ERROR 1045 (28000): Access denied for user 'dummy'@'127.0.0.1' (using password: NO)

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:

• 2023-07-19 07:09:19.761425753 +0000 UTC m=+1093451.531764466: ☑️ agreed by xhebox.