Added --enable-cleartext-plugin option to mysqldef

[Code-Hex]

What

I added --enable-cleartext-plugin option to mysqldef

Why

• We use Cloud SQL as MySQL
• We need to log in to a database with IAM database authentication
  • This method is required --enable-cleartext-plugin mysql option.

see: https://cloud.google.com/sql/docs/mysql/iam-logins#logging-in-as-a-user

$ mysqldef -u codehex -p $(gcloud auth print-access-token) -h 127.0.0.1 -P 13306 --dry-run database < schema.sql
2021/10/14 00:22:00 Error on DumpDDLs: this user requires clear text authentication. If you still want to use it, please add 'allowCleartextPasswords=1' to your DSN

$ mysqldef -u codehex -p $(gcloud auth print-access-token) -h 127.0.0.1 -P 13306 --enable-cleartext-plugin --dry-run database < schema.sql
-- dry run --

[Code-Hex]

Honestly, I thought the way was weird.
I'm wondering if the struct used to build DSN should be created for each driver. But I think that's too much for this PR, so I simply addressed it 🙇

[k0kubun]

👍 Thank you!