Allow DBaaS Operator to specify user authentication method on creation

[tobybellwood]

In MySQL 8.0, the default authentication method changes from mysql_native_password to caching_sha2_password which requires additional libraries to support client-side https://dev.mysql.com/doc/refman/8.0/en/caching-sha2-pluggable-authentication.html

We should consider being able to specify the desired authentication plugin to use for created accounts to minimise potential impact to users CREATE USER xxx IDENTIFIED WITH mysql_native_password BY 'password';

dbaas-operator/controllers/mariadb/mariadbconsumer_controller.go

Line 420 in 1541723

createUser = fmt.Sprintf("CREATE USER IF NOT EXISTS `%s`@'%%' IDENTIFIED BY '%s';", consumer.Spec.Consumer.Username, consumer.Spec.Consumer.Password)

Noting that currently RDS Aurora and Google Cloud still default to mysql_native_password, but others (notably DigitalOcean) may not
https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/Aurora.AuroraMySQL.Compare-80-v3.html
https://cloud.google.com/sql/docs/mysql/features#mysql8-authentication