server: fix authentication with MySQL 5.1 and older clients (#29732)

[bb7133]

cherry-pick #29732 to release-5.3

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What problem does this PR solve?

Issue Number: close #29725

Problem Summary:

handleAuthPlugin checks for the ClientPluginAuth capability but this didn't set the AuthPlugin causing readOptionalSSLRequestAndHandshakeResponse to return an error.

Note that this allows authentication for users that use mysql_native_password to work, but users using caching_sha2_passwords won't work.

This does now return a more specific error in case authentication with caching_sha2_password is used.

[dvaneeden@dve-carbon ~]$ ~/opt/mysql/5.1.73/bin/mysql -h 127.0.0.1 -u sha -P 4000 -psha
ERROR 1105 (HY000): ERROR 1251 (08004): Client does not support authentication protocol requested by server; consider upgrading MySQL client

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

Verify the connection with MySQL client v5.1:

sh> mysql -u root -h 127.0.0.1 -P 4000

• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

None

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• djshow832
• wjhuang2016

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Details

Commit hash: 2eeac32

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Details

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