backend: extract protocol processing from Authenticator and add a test suite

[djshow832]

What problem does this PR solve?

Issue Number: close #29

Problem Summary:

We need to mock a client and server to test authentication and command processing. To reuse the code of parsing and writing the MySQL protocol, we also need to extract the MySQL protocol-related code to PacketIO.

What is changed and how it works:

• Extract MySQL protocol related code from Authenticator to PacketIO.
• Define a testSuite, which creates a SQL client and a SQL server that connect to the proxy with TCP connection
  • Both the mocked client and server act like a real client and server: they parse data and respond to the proxy.
  • We can test various client capabilities, auth-plugins, and TLS configs using the mocked client and server. We create M^N combinations of the possibilities to test.
  • We test the correctness by checking whether the data received by the client is equal to the data sent from the server and vice versa.
• Test Authenticator using testSuite. testSuite will be also used for testing CmdProcessing.

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 (Don't forget to add the declarative for API)
• Has weirctl change
• Other user behavior changes

Release note

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None