Wire up valid_target_type function to column lister

[mathemancer]

Fixes #3706

Adds the valid_target_types array to the columns.list return.

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[Anish9901]

Looks good to me @mathemancer, can you please add a docstring for valid_target_types in ColumnInfo?

[mathemancer]

Looks good to me @mathemancer, can you please add a docstring for valid_target_types in ColumnInfo?

Done. good catch.