Add Connection Attributes for Handshake

[elemount]

Connection attributes can be useful. The default attributes as present in Connector/J, libmysql, Connector/Net are useful to identify which pid the client process has.
Looks like

mysql> SELECT * FROM performance_schema.session_connect_attrs
-> WHERE processlist_id = CONNECTION_ID();
+----------------+-----------------+------------+------------------+
| PROCESSLIST_ID | ATTR_NAME       | ATTR_VALUE | ORDINAL_POSITION |
+----------------+-----------------+------------+------------------+
|             94 | _os             | Win32      |                0 |
|             94 | _client_name    | libmysql   |                1 |
|             94 | _pid            | 6120       |                2 |
|             94 | _thread         | 4740       |                3 |
|             94 | _platform       | x86        |                4 |
|             94 | program_name    | mysql      |                5 |
|             94 | _client_version | 5.6.10     |                6 |
+----------------+-----------------+------------+------------------+
7 rows in set (0.08 sec)

This will help to do statistics.
Can we provide some info like _client_name = mysqljs ?

[sidorares]

@elemount it's implemented in mysql2, would be good to port to mysqljs/mysql as well

const mysql = require('mysql2')
const db = {
  port: 33306,
  user: 'root',
  connectAttributes: {
    foo: 'bar'
  }
};
const connection = mysql.createConnection(db);

>  SELECT * FROM performance_schema.session_connect_attrs WHERE processlist_id = CONNECTION_ID()
╔════════════════╤═══════════╤════════════╤══════════════════╗
║ PROCESSLIST_ID │ ATTR_NAME │ ATTR_VALUE │ ORDINAL_POSITION ║
╟────────────────┼───────────┼────────────┼──────────────────╢
║ 17             │ 'foo'     │ 'bar'      │ 0                ║
╚════════════════╧═══════════╧════════════╧══════════════════╝

implemented here - https://github.com/sidorares/node-mysql2/blob/03683f433f621e206814b837e46bc7f46ef45e0c/lib/packets/handshake_response.js#L118-L134

( looks like there is a bug and PLUGIN_AUTH flag must be set for CONNECT_ATTRS to work, need to fix that )