[Feature Request] Collect column names in create task

[bartvanb]

Problem description
In creating tasks, there are steps to do data preprocessing (extending and filtering it). These steps require the column names of the data to be known at that point, e.g. because if you want to compute the age given a date of birth column, you need to know what the name of that column is.

Desired solution

• When a user initiates a first preprocessing step, collect the column names with the /column endpoint.
• Alert the user that this is taking place and that they may wait a short while
• Handle case where the endpoint fails - error message

Additional context
Link to the backend implementation issue: vantage6/vantage6#778

[bartvanb]

This is already partly implemented. However, we didn't cover all features yet:

• If there is a function argument of type 'column', the columns still have to be collected when reaching the final step in task-create to set argument values and displayed as dropdown for the relevant arguments
• If new columns are created in the preprocessing section, they should be added to the column list

[bartvanb]

Closing this as it has already been implemented by now