Set library specific connection attributes

[dveeden]

Set connection attributes like _platform, _os, _client_name, etc. by default.

Have SetAttributes() update the map instead of overwriting it.

This makes it behave similar to other MySQL clients.

See also: https://dev.mysql.com/doc/refman/8.0/en/performance-schema-connection-attribute-tables.html

Wireshark:

Example:

package main

import "github.com/go-mysql-org/go-mysql/client"

func main() {
	conn, _ := client.Connect("127.0.0.1:4000", "root", "", "test", func(c *client.Conn) {
		c.SetAttributes(map[string]string{"app": "myapp"})
	})

	conn.Ping()
	conn.Close()
}