Can't use variables in SQL validate action column

### Version

Citrus `2.7.2`

### Problem

The `validation` section in an SQL action has the attributes `column` and `value`. You can use Citrus variables in the `value` attribute but not in the `column` attribute.

Referring to column names via variables is useful since they can be named more clearly this way. SQL column names don't always reflect the name used in the application domain, e.g. an XML field `lastName` may be stored in a DB as `name_2`. To make the connection clear for the test you can name the corresponding variables `${lastName}` and `${SqlColLastName}`.

### Example

```xml

...

```

### Solution

Allow dynamic variables in `validation`'s `column` attribute.