Support connect to both tls and non-tls server

[xuanyu66]

• Using tls=preferred, the server accepts a TLS connection from the client but is also okay if the client does not switch to encryption.
• upgrade go-sql-driver > 1.7.0 to use this feature for older version MySQL server Add API to express like a --ssl-mode=PREFERRED MySQL client go-sql-driver/mysql#1370.

[CLAassistant]

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