Add conditionals to joinTable column mapping

Thanks for the great work! Just a hope for a future enhancement. But basically, it would be nice to be able to add conditions to a joinTable, based on instance values or hard coded values on column names. I've used it in rails several times, and my projects where I use Grails mostly, the ability is sorely missed. See example below:

```groovy

class Employee {
static hasMany = [projects: Project]

static mapping = {
projects joinTable: [name: 'EMP_PROJ',
column: 'PROJECT_ID',
key: 'EMPLOYEE_ID', where: {->
[type_column: this.some_discriminator_column_of_this_instance]
}
}
}
```

Start by tracing Grails' joinTable column-mapping behavior; the issue provides no file or test names. Compare the requested instance-based and hard-coded conditions with existing mapping behavior, and consider the work complete when joinTable supports the example condition reliably with appropriate tests.

Written by the indexing model from the issue text.