Support mysql_clear_password if LIBMYSQL_ENABLE_CLEARTEXT_PLUGIN env set

[Matthew Peveler]

Allow the mysql_clear_password plugin if the LIBMYSQL_ENABLE_CLEARTEXT_PLUGIN environment variable is set. sidorares/node-mysql2#438 (comment) provides details for how implemention would work, but it has to be translated to the authPlugins scheme.

Useful links:

• https://www.thegeekdiary.com/understanding-mysql-pluggable-authentication/
• https://dev.mysql.com/doc/refman/5.6/en/cleartext-pluggable-authentication.html