MySQL: Support adoption of existing MySQL users when passwordSecretRef is not specified

### What problem are you facing?
When a `User` is created for a user that already exists in the database and `spec.forProvider.passwordSecretRef` is not specified, provider-sql does not rotate the password and `writeConnectionSecretToRef` ends up empty.

### How could Crossplane help solve your problem?
If `passwordSecretRef` is not specified and the user already exists in the database, provider-sql could automatically generate a new password, update it in the database, and store the generated credentials in writeConnectionSecretToRef.

If this sounds reasonable, I can work on implementing it and open a PR.