TiProxy is incompatible with go-sql-driver 1.3.0

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. Start TiDB and TiProxy
2. Connect to TiProxy with go 1.3.0

import (
	"database/sql"
	_ "github.com/go-sql-driver/mysql"
)

func main() {
	mdb, err := sql.Open("mysql", "root@tcp(localhost:6001)/test")
	if err != nil {
		panic(err)
	}
	err = mdb.Ping()
	if err != nil {
		panic(err)
	}
}

2. What did you expect to see? (Required)

No error

3. What did you see instead (Required)

Driver reports this user requires mysql native password authentication.

4. What is your version? (Required)

master.

The reason:

• Go-sql-driver sends the auth packet with mysql_native_password. But when TiProxy requests to switch auth method to mysql_native_password, go-sql-driver just reports ErrNativePassword. See https://github.com/go-sql-driver/mysql/blob/v1.3.0/packets.go#L495-L497. I think there's no way to avoid this problem on TiProxy side.
• Connecting to TiDB directly doesn't have this problem because TiDB won't send a switch auth request in this case.
• Go-sql-driver 1.4.0 works well.

Reproduce the failure by starting TiDB and TiProxy, then connecting with go-sql-driver/mysql 1.3.0 using the example in the issue. Read go-sql-driver/mysql v1.3.0 packets.go around lines 495-497 and compare the authentication handshake with version 1.4.0; done means the compatibility problem and a project-owned fix path are established.