backend: ignore parsing attrs error during handshake

[djshow832]

What problem does this PR solve?

Issue Number: close #283

Problem Summary:
TiProxy encounters an error during parsing attributes and it panicked.

What is changed and how it works:

• Add a Warning type and a user error msg
• Ignore the error during parsing attrs

Check List

Tests

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

Notable changes

• Has configuration change
• Has HTTP API interfaces change
• Has tiproxyctl change
• Other user behavior changes

Release note

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None