[ML] Import using existing index

[jalbrizio]

Describe the feature:
From the Data visualizer, the drag and drop feature only allows an import by creating a new index this would be more helpful to be able to select an existing index to add data to as another option to allow for multiple csv files to be imported into one index.
Currently, the Data visualizer requires the deletion of the index and then, concatenate all of the CSV's, then re-import the csv.
Describe a specific use case for the feature:
For example, if I have the office manager save a staffing csv each month they could then import each csv to have a dashboard updated with the latest monthly staffing totals.

[elasticmachine]

Pinging @elastic/ml-ui (:ml)

[jgowdyelastic]

This appears to be a duplicate of #31380
As per this comment, this is something we have deliberately limited so not to compete with other, more established ingest methods.

It is possible to generate a Filebeat configuration after uploading a file through the File Data Visualizer. This allows for further uploads to the same index using the same ingest pipeline.
This button is found at the bottom right of the page after file upload as completed.