allow to use arbitrary plugin as first auth method

currently only allowed connect method initially is mysql_native_password. Some servers can potentially prefer to start with custom auth immediately, instead of rejecting mysql_native_password and doing AUTH_SWITH_HANDLER sequence afterwards. Also some servers can be configured to allow 'plugin based auth' but not 'auth switch request' - those are two different capabilities flags

1. respect handshake packet plugin name
2. deprecate authSwitchHandler and rename it to be authPluginHandler
3. provide default handler for mysql_native_password

also need to think of something to make it easy to chain handlers:

const mysqlIamAuth = require('mysql-iam-auth'); // imaginary, does not exist
const mysqMyCustomAuth = require('@internal/customauth');

const pool = mysql2.createPool({
  authPluginHandler: combineAuthHandlers(mysqlIamAuth, mysqMyCustomAuth)
})

ref http://stackoverflow.com/questions/43448563/connecting-to-mariadb-with-nodejs-over-ssl-with-clear-text-password/43450396#43450396

Start by tracing the connection handshake and existing authSwitchHandler flow described in the issue, including how the server's plugin name is read. Compare the requested initial plugin handling with the current mysql_native_password default and consider the handler naming and chaining requirements. Done means custom plugins can be selected from the handshake while mysql_native_password remains supported.