backend: always set ClientPluginAuth

[disksing]

What problem does this PR solve?

Problem Summary:
node client cannot connect to server

test code

var mysql = require('mysql');
var connection = mysql.createConnection({
  host: '127.0.0.1',
  port: 3306,
  user: 'root',
  password: '123456',
  database: 'test',
});
connection.connect(function(err) {
  if (err) {
    throw err
  }
  connection.query('SELECT DATABASE();', function(err, rows) {
    if (err) {
      throw err
    }
    console.log(rows[0]['DATABASE()']);
    connection.end()
  });
})

the cause is that the client does not set capability flags correctly and the connection is terminated by tidb.

What is changed and how it works:

• always set ClientPluginAuth when connecting backend
• if client does not set ClientPluginAuth, log a warning

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 tiproxyctl change
• Other user behavior changes

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None