Bulk create contacts from a CSV file. The task runs in the background (processed by the task processor)
and its progress/result can be tracked through the returned task.

The CSV file must be comma-separated, with a header row, and contain the following columns:

Example:

contactCode,channelType,externalCode,name,language,proactiveEnabled,reactiveEnabled
34600000001,cloud_api,ext-001,Juan Perez,es,true,true
34600000002,cloud_api,,Ana Gomez,en,false,

Contacts for different channels can be included in the same file, one per row. Empty rows are
skipped, and empty cells are treated as an absent (optional) value. Invalid rows do not abort the
process: they are counted as failed and reported (up to 100 row-level error messages) on the task.
If a contact with the same contactCode and channelType already exists, it will be ignored.